By law a Thai child can own land in Thailand, but some land departments won’t make it easy if a foreign dad is involved. Plus once it’s in his name, you can’t have control over it without a Thai court’s approval. So if you want to sell the land in a few years, you’ll likely won’t be able unless you have a good reason that benefits the child, eg need the money for your child’s tuition. Also you won’t be able to get a usufruct I think as the kid is not old enough to sign off on it, so again it has to be approved by the court, and unless you pay good money for that usufruct, there is no advantage for the child, so the court wouldn’t grant it. It may be possible to get the usufruct from the seller first, and then your son buys the property with the usufruct already on it. I think it’s best to talk with a local lawyer who has good connection both with the land department and family court, before doing anything.