3 Yrs 4 Months Speech

Your pediatrician or school district can help you contact your local version of Child Find, which locally evaluates children for speech, motor, hearing and vision problems. It is free and will let you know if he's in norms or not. Overall, it sounds like he's a pretty normal kid, to be fixating on something he likes. My DD will talk your ear off about dinosaurs. Just keep talking to HIM about various things and see what he does.

Why do you want him to talk about varied topics? It sounds like he is a happy, normal little boy. You could take him to see some trains or to a boat show, he might get a kick out of that.
If you are seeking affirmation that he is perfectly normal, he sounds like most other 3 year old boys I've known, including my own.
Give it a few years, subtract trucks, add dinosaurs. Then another year, subtract dinosaurs, add Star Wars.
Enjoy!

He is 3, he is going to talk about what interests him and right now it is trucks. You can bring up all kinds of topics and that is great. He sounds academically on track so don't sweat the small stuff mom. You can always use his love of trucks to open up other topics by asking him questions: do cats drive trucks? why not? what do cats do? and gorillas? etc. He has a passion, go with it.

Keep in mind he has been accumulating all of your words for the last year or more. Soon he will have an outburst of words and you will be overwhelmed by all of his questions and chatter.

Kids do things on their own time. It sounds like he is talking, but perhaps not on various subjects. A pediatrician once said, they should have 200 words by the age of two. I think that is just a rule of thumb, as some will have more and some will have less. Hopefully he is beyond his 200 words by now.

Last year my husband and I were sitting in front of our daughters kindergarten teacher and she said, "She can't read". We looked at each other and then her and said in unison, "No, she can't". This was after one month of kindergarten. Now one week into first grade, she can't put the books down. She is reading level two books and will read for an hour each night.

I admit, comparing her to the rest of the class made us a bit uncomfortable, but she is fine. She just had to do it on her own time.

You can't MAKE him talk about anything any more than you can MAKE anyone talk about "varied topics."

First, people talk about what they know, have experienced or seen. He's only 3 - he hasn't had "varied" experiences, his world knowledge is extremely limited as is the world he has witnessed.

What is it that you want him to talk about? Whatever it is, you'll have to expose him to it in order for him to have something to say about it.

Perhaps he needs more kid interaction? If he has trouble communicating with other kids his age, you may want to try Early Intervention. My son has articulation issues with his speech, so we started services at the IU and it's really made a huge difference in how he communicates to kids and adults. Also, once he goes to preschool or kindergarten, I'm sure you'll notice a big developemental advance. If he's not doing preschool, it may be a good investment, even 2 days a week, so that he's interacting with/being taught by other people than your self.
Maybe you could go to the library and find some books about "new" topics. After we read/re-read books, my kids always have questions and talk about books over and over!

So if I'm understanding you correctly, your son speaks well, you're just sick and tired of hearing about trucks? My son is the exact same age, and I'm so there with you!
I find that I am less bored if we have more interesting information to talk about. For example, have you read Jerry Pallotta's Construction Alphabet Book? It's not your average babyish alphabet book. There's lots of interesting info in there that will be new to you and your son will love, and even some humor that you both will appreciate. We're big fans of his books, and they've helped us to branch out into other areas of interest. My son also loves Pallotta's airplane and boat and underwater alphabet books. It's a serious relief to hear him talking obsessively about airplanes and zooming them around once in awhile. See if you can first broaden your son's interests into other types of transportation.

I only let my son watch TV a few times a week, but he has seen some Bob the Builder cartoons and parts of the first Cars movie. This gave him the idea that his toy vehicles can have personalities and do more than just make zooming noises. We have lots of Cars vehicles that he plays with like dolls practically. They have friendships with each other, challenge each other, occasionally fight with or help each other.

I also go to the library on my own to carefully look for books with some type of vehicle in them that also have a good story-line and strong characters. We've enjoyed some books about brave little boats that go on adventures, etc... At some point he became really interested in different types of fish and underwater creatures after we read a boat book, so now we have branched out in that direction! Some books I choose just have a couple of pictures of, say, a school bus in the background, but it's enough to get my son to give it a listen. He usually likes the story regardless, once he listens to it. Anyway, good luck getting your son to diversify his interests! All I can say is, trucks are annoying, but at least it's not princesses! Let me know if you find anything else that works well!