Falling asleep in his food

Ever since he started preschool, Micah has been a less-than-consistent napper. He used to be a one nap for three hours kid, which was fantastic. If we can get him to sleep now, he still usually sleeps for 2 or 2 1/2 hours, which is great. The problem is that he just won't do it every day. I've learned that in parenting, you pick your battles and sometimes I'm just not willing to fight to get him to nap. In the last two days, although he hasn't slept in his room, Micah has found other places to just pass out when he got too tired to stay awake. First, actually in his food...and then sitting in the chair by the computer, watching garbage trucks on YouTube. I had to wake him up and move him (after I took the picture, of course) because he was swaying so much while he was falling asleep that I was afraid he was going to fall out of the chair!