Are you using nice ripe fruit? If using fresh, try not to get the parts close to the pit.
They may also taste less sweet than he would find them if you as an adult are more accustomed to sweeter tastes. They might taste plenty sweet to your LO. Definitely do NOT add honey for LO's under a year. If baby objects, perhaps add a TINY TINY bit, like a couple drops, of fruit juice to the puree, but seriously, not much, and be sure it's juice and not "juice cocktail" or other sweetened mixture.
For constipation you can also try prunes, or prune juice in with other liquids (some people add it to a bottle of EBM or formula).