As a guy in my early twenties who went through this and know other people who did, I would suggest that he is really looking for any excuse to ‘have fun’ by doing as he pleases. When someone is brought up in a place where judaism implies that secular films/music are forbidden by law, you have to dress in black and white, you have to go to a yeshiva at the expense of secular studies (which would lead to work) etc. then religious judaism leaves the pure rational halachic boundaries and becomes a cult based on modern traditions with the single aim of who can be holier/stricter. This is the excuse he needs; why should he keep things that appear to make no sense even within judaism.
Do you want you son to be a religious jew who follows the shulchan oruch, one who follows the corrupt philosophy of modern day haredi’ism or be secular?
I would humbly suggest that instead of trying to send him back to haredi yeshiva where irrationally and stringencies reign you should send him to a modern orthodox yeshiva/uni where he can have a chance to be able to intellectually rationalise judaism in the current world. Where he can feel that he is not stuck in a medieval religion but one that is relevant today.
You can’t force faith on someone. You can give them a chance to learn if you put them in an atmosphere where they are encouraged to rationalise what they are doing.