Be supportive and loving, but also do not tolerate actual chillul Shabbos in your home. It is one thing to love the person and accept him or her, but you cannot allow it to be rubbed in your face, either, just as you cannot allow them to bring tarfus into your home and eat it in front of you off your own plates. there are line that cannot be crossed and they have to accept that, the same way they want to be accepted by you. Love the sinner, hate the sin, ONLY. Always keep the lines of communication open.
What seems OTD to some, is laughable to others. If your son is wearing jeans and a t-shirt – THAT doesn’t put him OTD (rebellious perhaps), though I am sure many people would view that behavior as being if not OTD, then on the road to OTD (and they could be very wrong about that and actually be marchik that person by treating him as if he were OTD). But if he is eating in McDonalds, smoking and driving around on Shabbos, then he is OTD and that must be faced.