the first task you work on, will require a lot of creativity from your side, as you know him better than any of us, but iut is the great time that starts for you both!
Here are couple examples:
MY son once started responding on any of my questions 'yes-no'. It lasted for months: I ask, for instance, are you hungry?" and he responded: "I am hungry, not hungry."; "Do you want to go for a walk?" "I want to go, not to go", and looked at me very seriously and inquiringly.
Another situation we had, was that once he decided he needs two of everything. He wouldn't start eating one slice of bread, or even his favorite pie. He needed two. He just held the one in his hand and said: "two." Then, I took it from his hand, broke the slice in two, handed him two, he held one in each hand, and happily devoured both, munching from one and another one simultaneously. Same with toys. He came to me with his little toycar and announced "Two!" So, we walked to his toybox, and found another one, about the same size. With a car in each hand, equaling two, he could play alone or in his brother's company for a long time, no problems.
You will have some interesting situations, obviously.
Do not get frustrated. As creative as he is, he expects you to be even more creative.
I have no clue what would work, but you will need to try many things.
Important is though: do not approach it straightforward, as you tried it already and it did not work.
For instance, he starts whiling, why don't you sit on the carpet and shart quietly whining also (peeking at how he reacts of course). As this is totally unusual behavior for him, the hope is, he will quit his whine, or at least become curious. If he will come closer, you may ask for his help: "mom is so sad, she needs a hug, can you help her?"
if he starts whining, start doing something (not addressing him AT ALL), what he really likes, in the same room: take his legos, start building some castle, and talk TO YOURSELF: "I like to build this castle, but if _____(Your son's name) was here, it would be so much more fun to build it together! But he is not here, so I will have to do it alone." and see how he reacts.
If you DO NOT PAY PARTICULAR attention to his whining, he should eventually get bored with it.
if you say, once he starts whining: "oh, little boy, I do not know who you are, but I am looking for my son ___(his name), and I miss him so much! You even look almost like my dear ______, but your face is so much different, I don't think this is my son. My son always smiles, and I have such a great story to read him, I need to find him right away!!! Maybe you could help me to find him, little boy?" and if you walk around a little, he will probably follow you, out of curiosity, and once you get into the room with the mirror, start looking inside the mirror: do it very carefully, attentively, inquiringly: see what he will do. I believe he will come to the mirror too, and it won't be a whiny face... THAT"S when you discover him! "Oh, here he is! Can you come out here, I need to hug You, ____, please!" and finally turn around and find him in this world, and express your surprise how you didn't notice himn here before!
You know what I say? None of these described above situations might work, but if you quit paying straightforward attention to his whining, do something absolutely extraordinary, to catch his attention and redirect it away from his 'sadness'(something jaw-dropping would be even better, you name it!), make life interesting for him to the point that he will forget whining, and always leave him a chance to approach you 'through the other door with a smile', you will be both fine pretty soon.
He is definitely trying out what is poossible with you, and you need to outwit him. Once he will see whining does not work, but not whining makes life so much more interesting, you are on!
Be VERY creative, explore your son's soul and mind, like he explores yours. And, do you not get frustrated, this will make YOU cry, and then your creative mind will not have enough energy to think about new ways of communication :) !!!!
With Potty training:
I responded about a month ago to one mom with a 3 years old girl. if I may, I just copy-paste you my response, and good luck, again, M.:
one VERY UNPLEASANT experience taught my daughter immediately:
I cannot suggest it, as it is for you to decide, but 3 years old is definitely ready to understand and do it right. She might feel it comfy not to bother to run to the toilet, but it's really time to 'grow up' :).
So, this is what I did: no diapers, regular underwear and stockings or pants on.
Once the clothes got wet :(, I helped her to change, but I tied these wet stockings to her tummy on top of her dress. Well, they kind of stink. She did not like it at all. She quietly suffered through about half an hour, then I took the wet stockings off.
Ever since, not one single time of such a problem occured. She knew where the pot is, and she 'helped herself'
Good luck to you!