Tiffany,
Yes. Changes in behavior, beliefs, actions can and will happen. And especially when they are new to recovery, they will seem to travel around the globe and back as they work through their emotional triggers.
It is very possible that he spent a lot of time examining his beliefs and spirituality while in rehab. Especially if it was a 12 step program. In a 12 step program, you turn your will and your life over to God as YOU understand him. We often speak of a HP who can restore us to sanity.
While in treatment he probably saw people with different beliefs and found that "God" was in all of them. This might have been a new concept for him.
He is probably just doing some soul searching. As adults we often take a look at the religion our parents gave us and then have to make a personal decision about it instead of just taking it on their word.
But it could be that he is just looking at some of the principles of Buddhism, not so much taking it on as a religion.
Whatever his reasoning or choice it is his. If its where he finds recovery than more power to him.
This might be one of many things you find are different in him as he is searching for strength in recovery.
I have found by talking to other Al-Anon friends that their recovering addicts seemed to be fanatics about something one day and it changed the next.
The journey to recovery can get a bit intense and you see it in their thinking, but it's theirs, and again, whatever it takes. Try not to let the changes frighten you.
And I say this with experience and kindness,
If you get out of his recovery, (his thinking and beliefs, as well as his actions) and focus on your own, you'll find a lot more peace in all of this.
I know people have suggested going to a meeting that focus's on being In a relationship with an addict.
It made all the difference to me. and allowed me to Let Go of my son's recovery.
glad he is on the road to recovery,
Cathy