I had a similar situation recently and posted about it. Isabella & Jasmines Mum gave me some good adivice that I am still using. My DD's are 17mths old (15mths correcte) by the way so similar to yours.
Guidelines I got were, in order to keep a 7pm bed time you need LO up and active for at least 4-5hrs before hand.
So you could get him up at 12, give him lunch, try putting him back down at 2pm for 1hr and then keep him up till bed time.
If he wont' go down at 2pm try 2:30pm, 3pm etc. Assuming he does go down at any of these times though means that you might have to move bedtime out a little.
I was always stressing about the time and now I just try to make sure that A time is at least 3.5-4hrs in between naps and it seems to work (my two couldn't do 5hrs).
Trisha.