M.F.
Hi T.,
I would just embrace the fact that he prefers his thumb to the pacifier and enjoy it. Both of my boys used pacifiers and my daughter is now a thumb-sucker. She never took to the pacifier, and it has proven to be so much easier. She is 20 months now and she only sucks her thumb when she is going to sleep or very, very tired. You will likely find that he will do it less and less as he gets older.
Congrats, and good luck!
M.