Just discovered both my ex and I are FAs: should I tell him?

I agree with tnr9 . The only time I've experienced people successfully addressing their attachment issues was when they came to me to really talk about it. It never worked when I brought it up to someone else, and especially never when I brought it up to a romantic partner / ex. One ex in particular thought it meant I was implying he was damaged and trying to change him, and in response he dug his heels in harder to avoid his own issues. People change for themselves, usually because it's more painful to stay the same than it is to change. They don't change for partners, no matter what Hollywood movies tell you.

Think of why you're looking for professional help now. Maybe the catalyst is hurting over this breakup and wondering if things could be different. But I bet underneath that, you've been hurting and lonely for a long time, wondering why your romantic relationships seem chaotic instead of smooth, and you're at a point in your life where you're open to healing yourself and change to see if you can feel better if you do something differently. If someone told you to go do that a few years ago, would you have been receptive to hearing it and started looking for a therapist, or would you have brushed them off?

I think the fact he's 51, a middle aged mature adult, and still hasn't concluded on his own that this is introspection and self-work worth doing means he likely won't be interested in working on it right now. I used to do what you're doing, making excuses such as he's got a difficult new job and you need to be patient while he adjusts and then he'll show up again for the relationship later when he's under less stress, but I've since learned not being able to manage stress, time, or conflict is a gigantic red flag if you want a long-term relationship. Life throws stress at you all the time. Someone who can't manage their work-life balance or stress levels and lets separate issues interfere with your relationship and connection is not going to be supportive of you or see you both as teammates at the times life is tougher. That doesn't mean people can't get stressed out or shouldn't be allowed to get stressed out, but it does mean they should have the maturity to deal with it without taking it out on you and the relationship. So do not let a stressful job fly as an excuse to break up: the real reasons for the break up are the person doesn't want to prioritize a romantic relationship and doesn't deal well with stress and conflict resolution. Teaching him about attachment theory isn't going to change those reasons, and it may take him years to learn to address those things better IF he's even interested in making changes.

Focus on your healing and yourself right now. It's commendable you've started therapy. A lot of people can't get themselves there. It requires a lot of strength of character and resilience to take the steps you're taking, and you should be proud of yourself for moving in the right direction. Don't get distracted by someone else's stuff.