I think it all depends on how he grows..
If he's long, I'd go with 18 or 24 months, maybe even 2T for the winter. There are kids in my
PR that are in 2T and 3T. Mostly the tall kids. If he's short, I'd stick with 12 months.. Adriana falls into the short category... She's almost 19 months old and she can still wear 12 month clothing.. She is slowly starting to transition to 18 month clothing, but 24 month clothing is out of the question.. Just way to long and big..
I learned last year not to buy lots of clothes in advance.. Adriana was a late September baby and I thought that she would be wearing size 12 months during last summer.. I was wrong.. She was only in size 6-9 months. All of her 12 month summer clothing was way to big and she never wore them. But 12 month clothing fit wondefully in Fall/Winter/ and Spring. If I do buy cloths in advance, I buy the size for her age..