having trouble with 3rd nap :-(

I've been following the 90 min to 2 hr rule for when to go down for a nap and it works wonderfully! The only problem is when it gets to be 5/515 I'm not sure if I should put him down again since his bedtime is 630/7 :-/ should I put him down and make bedtime a little later or keep him up? He gets super fussy and grouchy tho around 6 . I get overwhelmed then and sometimes end up putting him down earlier because of this. We still don't have a fixed schedule when we get up in the morn so that's why sometimes the schedule works out like this.