As you've been so badly hurt in the past it is only fair and reasonable that you ensure that these communications between your lover and his ex either cease or remain harmless. It is generally not usual to maintain a close relationship after a breakup, unless he can truly satisfy you that it's entirely platonic, ie non-emotional and non-sexual. Such relationships with past lovers do occur and are seemingly pretty harmless. You want an exclusive relationship in return for your committment to him. If those two are just friends now, then your relationship is not jeapordized at all.
If he insists that his ongoing relationship with his ex is quite safe and above board, with no intimacy, ie, they are just friends, then you have no option other than to entirely trust him as part of your love. This will be a big step for you.
At his age I expect that he now knows what he wants in a woman, and that's you! He could have re-chosen his ex but wanted you instead. Enjoy the relationship without reservation if you are reasonably satisfied with his integrity.
In any event, because you have some doubts, it seems prudent to continue to wait until you become appropriately satisfied about his love for you and only you. Just don't let past pains paralyze you. Time runs out eventually!