Hmmm, difficult one but as you say the A times are really low for his age, but they are just a guide.
Wrt the short nap, I would try and resettle for a max of 10-15mins, if he doesn't look like he's going back down I would get him up and try a slightly shorter a time to the next nap. Here is when it gets a little difficult. Some lo's like mine can do a full A time after a short UT nap. First I would try 15mins less and see if not you'll know for next time.
I would try w2s on the naps hun. So at around 30mins, go in and rouse him gently by stroking his cheek try not to wake him fully (but it does happen), wait for him to sigh or his go breathing to change slightly then back quietly out of the room. As I said though, as the A times are a little low it may not work and you may find he'll wake up completely. But worth a go?