the onesie doesnt have any tags or labels or anything? you cant remember any markings on the doll? pretty tough to find! you might have to settle with just getting a similar baby doll.
i know you said plush feet but could it have been a JC La Baby?
https://www.amazon.ca/JC-Toys-Washable-Children-Berenguer/dp/B002APDYV0
if not, they're cheap ($15) and sound pretty similar to what you're looking for.
The onesie reminds me of a baby doll my daughter had when she was little (16 now)
I had gotten it at walmart and I believe at the time she was around $10
You might want to check there. They have quite a few adorable, yet, inexpensive babies that he might like to "adopt".
Kudos, btw, for your son loving his doll baby. He will be a good daddy one day for your effort.