how old is he?
if he can handle 5 hours A time and gives you a 2 to 2.5 hour nap then i think he is ready to go to 1 nap. but bedtime will have to be earlier until he is adjusted. when he goes to 1 nap i would aim for a 12 hour day - 5 hour A followed by 2 hour nap followed by 5 hour A means a 12 hour day so bedtime would have to be 7 pm if he wakes up at 7 am. kwim? (i would aim for nap from 12-2pm but let him sleep as long as he wants, don't wake him up)
be careful with it, when we switched we did 2 naps on some days as she would get so tired, it was like catch up sleep, and then she would do 1 nap again. just watch his cues. some days he may be able to handle 1 nap and some days he may need 2 if he is doing something very stimulating that day, or teething , or sick, or just tired from a week of 1 nap.
when we switched for a while actually my lo did a 13 hour night and an 11 hour day - her bedtime was so early but we had to switch because of daycare, i would have let her do 2 naps a bit longer if i could have.