Most Helpful Guy
If he has otherwise been loyal to you, I would advise you to forgive him, but make it clear that you will not tolerate any slip-ups.
What he did with his ex is obviously a problem, but you can evaluate him as a boyfriend based on your time together with him, so what he did with his ex is less significant.
Most Helpful Girl
Ok so now you just make a decision whether or not u wanna continue seeing him.. I know you love him but if you can't trust him or get that thought out of your head or if you're simply not ok with it, which understandably you aren't, you'll have to move on.