Hi, frannieandejsmom. Exciting times! Our son took the MAP recently. It's my belief that it is not timed by design, which should always allow for restroom / water breaks. Our son (5) was given it in two sessions also. He seems to have sped through it, but I think that even taking time your son will probably take no more than an hour each time.
Based on the advice of someone here, I made sure to ask that he not be rushed at any point (again, that wasn't an issue for us but I thought I should mention it). Also, prime your son to resist if someone tries to rush him because he's the last one left.
Good luck! It sounds like you can't lose much either way. Get EJ pumped up at the chance to do his best, continue stressing that he needs to focus even on the easy ones and that he will get some wrong, and keep your fingers crossed.