B.C.
My son is 4, almost 5 and we went through a stage like this. It is frustrating, especially with other children around. Try allowing him to "pick" his own clothes or lay out two or three outfits and let him pick which one he wants to wear.
There are a lot of changes going on for him and he is probably frustrated and does not know how to express it. Make an effort to talk to him, maybe take just 10 minutes every day that is just his. read with him or play a game with him. I have gone as far as talking with my son and asking him to tell me why he is upset- if he can explain it to you that may work. If the separation is recent, he is missing his dad.