Here's the result of my PubMed search on trigger thumb in kids. There remains no concensus on what is the best method of treatment. I've referred him to the local hand surgeons – who specialize in adults. They suggest no spinting and observation until at least 1 year of age. This seems fine, but several studies suggest that spinting may actually help.
In the absence of a good evidence, expert opinion may be my best bet. I think I'm going to call Boston.