Yeah, you don't want to take him out if the weather isn't nice.. the last thing you want is him getting ill! I found the last A before the catnap was the longest one (that is, before we dropped it) - but if you want to keep it at 2hrs, see how you go....but I think even if he falls asleep by 5pm and takes the catnap from 5-5.30; you can do bedtime at 7.30pm, which is still a reasonable bedtime. Remember, during the transition phase, it is not uncommon for babies to have longer days and do shorter nights.
It's funny hey... when you finally get 2 good naps, they start to fight the catnap but he will still need it until you are able to push his A's around 3 hrs. Don't be in a hurry to do that though - just keep on offering him the catnap and if its waaay too late for it, then an earlier bedtime is a must! I used to put mine down at 5.30pm some night to overcome overtiredness. Or if he does take the catnap and its getting late, that is, it will push the bedtime too late, then you can try waking him up say 30mins into the nap (or maybe even 20mins) and you will be able to protect bedtime.
At 5 months, 2.5hrs A time is about the average so if he's taking a good afternoon nap on that A time, then leave it at that for now. It may have just been that his morning nap was too long, which was preventing him from taking a longer afternoon nap.
Fingers crossed his afternoon nap went well