Unfortunately, only the child will choose. The only way to "force" it is to have the thing with your son at all times, and he will just become accustomed to it. I did this with daughter #1. We kept a certain little bear in her crib and eventually she needed it. Luckily, I got 2. Then with my second daughter, I tried the same thing but she (now she's 5) decides each night what she will sleep with. It is not always the same thing. My nephew chose a very large white bear to cherish. Bummer. It was NOT an easy thing to wash! My friend's daughter chose mommy's bathrobe. She cut it up into smaller pieces and made kind of mini blankets out of it.
The only thing with loveys is that they get lost. Especially when the child is 2 or 3 and is carrying them everywhere. They get lost in your car, at a friend's house, at the library, at the playground. If you want him to be comforted by something, just be prepared to have the thing around when he needs to be comforted.