AITA for telling my 15yo he has to see a therapist or lose his phone and activities until he does?

I have six kids, ages 6 to 17, one of whom we adopted three years ago. My 15yo Marcus has always been the most strong-willed of the bunch which honestly i respected, but over the past eight months something shifted. He snaps at his siblings constantly, slammed his door so hard last month that the frame cracked, told his little sister she was "the reason this family is exhausting" when she asked him to pass the remote, and completely shut down any time i try to have a calm conversation. I get that being 15 is genuinely hard and i am not trying to minimize that. But we have a big household and the way he moves through it right now is affecting everyone, including our adopted son who is already working through his own stuff and really doesnt need extra tension in the home.

I sat Marcus down last week and told him i think talking to someone outside the family would help him, not as a punishment but because he clearly has things he cant process with us and thats okay. He went completely cold and said i was "pathologizing normal teenage behavior" and that i was the problem for not just letting him feel things. He hasnt spoken to me properly since. My older daughter thinks i should back off and give him space, my husband agrees with me but says maybe the ultimatum was too far. I genuinely dont know if pushing this makes me the asshole or if i'd be failing him by dropping it.