What works for us, although we get about 11hours o/n & 1.75-2.5 in the day, is to be more active, esp between nap wake up & bed. If more running around doesn't work I guess you need to decide between cutting the nap back or later bedtime, as his total sleep is pretty much on par for 15mo (13.75 for 12mo & 13.5 for 18mo)