It sounds like you got spoiled with your first! Waking up only once during the night is totally normal for a five-month old, and much better than a lot of babies I know! Many children continue with at least one night waking well up to a year old.
I know you are hearing at a baby "should" be able to sleep through the night. I would recommend you do your own research about this, because this statement is extremely misleading. The medical definition of "sleeping through the night" is one five-hour stretch. Your son is certainly doing that and more.
It may be that your son needs to learn how to soothe himself back to sleep. "The No Cry Sleep Solution" is a wondeful book. It is packed with ideas for setting up bedtime routines and helping babies learn to get themselves back to sleep without leaving them to scream in vain for their mothers. You should be able to find it at your local library, or most bookstores have it in the parenting section. It helped us get my oldest sleeping through the night when she was 1 year old. One of the main recommendations that she offers is getting babies to sleep earlier. If your son is screaming through his bath and bedtime routine, it sounds like he is totally overtired. One of the things that we learned from this book is that by the time baby is yawning, rubbing his eyes, fussing, etc, he should already be in bed. Keep an eye on him for these signs. Whatever time he starts should be when you aim to finish his bedtime routine. Slowly adjust your bedtime until it finishes at that time. It's not easy getting babies in bed early, (especially when you have a toddler to deal with) but it is so worth it to have a happy, healthy well-rested baby!
Despite what some will tell you, it may also be that he is hungry, in which case there is nothing you can do about it until he gets bigger. Their little tummies simply may not be able to hold enough to get them through a 10-12 hour stretch. If you try some "sleep training" methods and they don't work, you might just have to give him some time to grow up. He is still a very tiny baby. Both my daughters were up once a night until they were about 11 months old.
Best of luck,