From my experience with my ds#1, waking up about 45mins - hour after bedtime, is actually due to overtiredness. which if he only gets 2 x30min naps during the day, would most probably be the case. At 7.5 mo, you would be expecting him to nap about 2.5-3 hours in total being 1x 1hour 7 1x 1.5-2hour is normal
So maybe if you can't extend the naps, try giving 4x 30min naps. His awake time between naps should be about 2.5-3hours if he does long naps, it would probably not be more than 2 hours if he does 30min naps, otherwise he is over tired going down for naps