It could be a phase, but it could also be something physical. Before just assuming it is a phase, I would make sure there is nothing physical going on.
When my sister was young she didn't like to poop either...but my mom eventually took her to the Dr. and they found she has these tiny tears around her anus... so it actually HURT to go poop. And then of course she would hold it, become constipated and then it would be worse.
The tears eventually healed, but we had to make sure her poo was very very soft so going would aggravate them anymore.
I do think there is something to be said for not making a big deal about it though. Toddlers are learning they CAN control certain things. If they don't do something it will get a response. I would talk to your doctor and make sure there is nothing physical going on. Then, depending on what he/she says just don't make it a big deal and ensure he is getting things that will make his stool soft. Make sure he drinks plenty of water as well and stay away from binding foods.... bananas, cheese, etc...