What I mean to say is unless you really think you can win that argument without further bruising on either side, then maybe better to let it go. Where he is headed is hard to say; people turn around, many OTD have turned around and for different reasons, and some after a short time, some after a long time. But you do have to deal with what is in front of you now, and that is his desire to change the color of his shirt, as a statement of affiliation. At this point, prioritizing which issues are most important is probably a good idea.
When a kid goes OTD and the family lives in a community with strong standards, there is that “other” relationship that gets changed dramatically, and that is the relationship between the parents & siblings and the rest of the community. There is a lot to be said on this topic, and it should be discussed openly in my opinion, with hadracha from our leaders. In the meantime, it may be wise to accept that there will be a change in how you view yourself & family vis-a-vis your community if you suspect there would be strong disapproval, and again, clarify your priorities. One thing I have found comforting is to remember that not only our spouses are bashert, but also our children are bashert to us.