Is it permissible to temporarily adopt my son, who has autism, in Germany so that he can complete his treatment and studies and be protected, and then cancel this adoption?

If your son needs treatment that is unavailable in your country, and his going to Germany is hoped to cure him while being safe from deviance, then there is no harm in sending him to his maternal uncle for the uncle to register him under his name until his conditions improve, bearing in mind that he is not a mahram to his maternal uncle's wife or daughters. This is not deceit towards the state, as there are laws that permit official adoption and the resulting treatment and rights. Once the need for his treatment and study is over, he must be returned to you and his papers reinstated to their correct status. This action is a concession due to necessity, and if the necessity ceases, it is obligatory to return to the correct, lawful situation. If the matter is fraught with risks, such as corruption, deviance, or interaction with the maternal uncle's wife and daughters, then one must deliberate, study the matter, and consult experts.