Well, I'd say if he's playing happily in his crib then I'd keep him there until the next feed. As for when his next nap starts, I'd recommend keeping him up to within a half hour of his next nap. He's 5 months and shouldn't be drastically overtired by then. If he takes a 45 min nap, then it's 3 hours until the next nap; put him down at 2.5.
The reason for this is to keep with the biorhythm. It is set when he wakes for the day, and he's been use to taking naps at certain times of the day, so he's more tired by then. You still may get a 45 min nap for the afternoon, but at least you're keeping with the pattern you've established. Hopefully he'll fall back into it, unless it's time to keep him up a little longer between naps

*HUGS*
Sonya =P