If he sleeps 3.5 or more during day it seems a bit too much for an average sleep needs kid. So if your son is not a HSN, his night wakings may be a result of a too much sleep per day. To help him go through the night I would probably go for either waking him earlier during day and lengthening his day or increasing his awake time and cutting naps to 2-2.5h per 12h day.