I know this can't be the easiest situation but it can be remedied.
Your 2 year old is ready for potty training. Any child that can clearly indicate his diaper is full can be taught to use the toilet.
With the introduction of the new baby, your two year old may be feeling a little left out in the middle of the night especially since he has just begun adjusting to being not so close.
Keep the lights out or extremely low the nighttime activities, reduce fluid intake a few hours prior to going to bed. Introduce going to the bathroom before going to bed, this may reduce the need to urinate in the middle of the night.
Be loving and consistent with the methods you are using that are working for getting him to bed and to stay in bed. Try some of the new advice you will be given. The tools you have to use the quicker this situation can be positively and effectively resolved. He is probably testing your resolve and his limits with you since he is two as well.
Hopefully this information is helpful to you.