Some embassies and consulate will issue a multiple entry tourist visa for being the parent of Thai.
Penang was doing them before covid 19 if you could show about 200k baht or its equivalent in a bank. Their website has no info about the requirements for a visa at this time. You could contact them about getting one. Phone: (604) 226-8029, 226-9484, 228-6784 Email:
[email protected]
You can extend each 90 day entry for 60 days at immigration to visit your son. Doing it that way would make easier to get the single entry non-o visa and you would only need 3 of them for total stay of more than one year,