my D was accepted to OCU the summer before last and we had the same dilemma. We called OCU to try to get a little more time to decide. She was eventually waitlisted at MPulse....but she was accepted to NYU-Tisch Cap 21 earlier, which was her first choice so she was able to then call OCU and let them know to let her spot go.
You could contact OCU and let them know you are still waiting to hear from another program and would like to delay making the commitment, you could just accept the spot at OCU (it is a fantastic school) or you could just lose your $100 deposit.
I would not count on MPulse, it might happen but all of these programs are competitive to get into....even the summer programs.
BTW- D is at UC Irvine now and has several classmates that attended Tisch summer, and MPulse---they all feel they had a great experience, so wherever your son goes-I am sure he is going to have a great summer! Congrats to him.