Your son seems to be having a (post-traumatic) habitual dislocation of the inter-phalangeal joint of thumb. Definitely not a common scenario! From what you stated, it seems that the frequency of the spontaneous dislocation is decreasing. What is important is to make sure that the joint is fully reduced and maintained in that position (if possible). If he is not complaining and has full function, and the joint is mostly properly reduced, then it is best left alone. Any complaints, along with loss of function (as compared to other side) would force an orthopaedic surgeon (pediatric orthopaedic or a hand surgeon) to consider surgery.