We try to keep the feeds pretty evenly spaced apart, so if he wakes after an hour's nap then you would wait to feed for a bit -- using activity to take up the time. Of course, you must be careful not to let feed time run into next nap time, so you probably won't be able to maintain a 4 hour spacing with a short nap. Also, when he short naps he probably won't be able to handle quite as long of an A time. Usually shortening by 15 min is enough to help them not be too OT for next nap.
Now, my philosophy was always if DS woke before 1h30 then I would try nap extension for 20 minutes. Every LO is different as to how responsive they are to nap extension techniques. So give it a try for a few short naps, but if that doesn't work, then you can decide to just let him short nap on those occasions.
If he's starting to short on a regular basis (at least 1 nap/day) it's time to examine A times again! At this age, they typically are ready to increase As by 15 minutes every 4 weeks.