Well done on confronting him OP! Now definitely take your time to process all this and work out what you want to do next. Don't let him dictate any time scales or pressure you in to sticking with him. And don't delete the evidence, especially if he makes it a condition on working things out.
If he's been working out and improving his appearance etc. that could have started out innocently, as a constructive way for him to handle the decline in your sex life. That is, when one partner still desires sex with the other, but the other partner's desire has waned for whatever reason, it's only natural that they feel the issue is with them no longer being sexually attractive to the other person. Even if there are a whole host of other legit reasons!
However, him working on his own fitness and appearance could have also involved you. If he wanted to change his look up a bit, he could have at least asked your opinion on what you thought looked good on him (ultimately, the clothes would be his choice though). He didn't need to invite you to the gym with him, but he could have suggested there be something active that you do take part in together (like a yoga class or something).
In fact, working on things like that together should definitely be something he should have suggested if you also didn't feel that great about yourself and that's having some part to play in whether you're up for sex or not. I'm speculating a bit now, and please do not take this as me suggesting that you are partly blame for your P's shitty behaviour - his actions are his own fault only. You have done nothing wrong! - just sometimes people can go through a phase of not feeling attractive or desirable, even if it's blatantly not true from our partner's perspective. And no amount of "but darling, you are still the most beautiful/handsome person on this planet and beyond" can help with that sometimes. Idk if that is the case currently for you, since it sounds like it's more that life in general is giving you a kicking atm, but I've definitely been there before, and know of others too so it's not an uncommon thing...
But when one partner is feeling like that, or even both, it is not right for the other to go seeking validation and gratification elsewhere before working on those issues together. If one partner is feeling down on themselves and not feeling at their best any more, both can work on building them back up and regaining self-confidence together.
Long story short: your partner decided to feel better about himself and chose to work on his fitness and image (fair enough). But instead of talking to you properly about how he felt, or including you in the whole self-improvement kick he was on, he kept it to himself and gave other women (emotionally or physically) the benefit of his newly regained self-confidence.
He's been an arse.
Take a break from him while you work out what to do, treat yourself and your DC, pamper yourself, do whatever you want or need to in order to genuinely feel good about yourself. Good luck OP!