My daughter (who is now 2 1/2 ) did exactly the same thing for a few months when we first started TT. I put it down to her having an issue with the release of her urine. She understood what the potty was for, she was interested in what mum and dad did on the toilet, she was aware of her bodily functions (i.e she would come and tell us if her nappy was wet or if she'd done a poo). While this was all a great forward step, it still didn't equate to her being ready to let go of her wee on the potty!! She would sit and scream the foundations down, and almost make herself sick! So we stopped trying for a while, tried again in a few weeks time. It took several attempts over the course of probably 4 mths before she actually managed to wee on the potty.
While i understand the idea that TT is not supposed to be a "rewarded behvaiour", i think that in the initial stages it can go a long way towards helping. The children are often a bit daunted and sometimes frightened of this new bodily function, and some positive re-inforcement can help smooth those issues away for them. I have used either sugar-free treats, favourite snacks (eg fruit salad, carrot sticks, sultanas etc) and special activities to help my kids to associate TT with being a positive function. Showing them and talking about how proud we are makes them feel more 'grown up' and indepenent. I don't give my daughter a 'treat' every time, but i make sure that throughout the day or at the end of a very good TT day, that she knows why she is being rewarded. Every family has to do and try whatever is best for them selves and their own beliefs/circumstances.
I would say your son is not quite ready to go it alone, so to speak, but perservere on and off over the next few months and he will gradually become accustomed to it. I had the same idea about getting my daughter TT before the birth of my baby in September.....HA!!! So much for that. Then when the new family member arrives, its not advisable to try too much TT, as your little boy will be emotionally overloaded with accepting and loving his new sibling and asserting his position in the new family order!
Hang in there....it will all happen eventually!