It would be wiser not to trust someone completely. People change. His priorities in the past had been his main focus. Now, he's realizing he also wants a relationship with you. As long as you are the his main priority and not his other obligations, then you can know that he loves you. From what you've described it seems that he is someone trying to figure out what he wants and in what order. You've been forgiving and that should not be taken for granted.
You would have to look for his behavior and not always what he says. Actions are what provide tangible proof of someone's intentions. Having had several break ups within a year or so is somewhat of a red flag. Is he coming to you because you provided unconditional acceptance/affection while he was trying to figure out what he wanted or is he not ready for commitment at this time of his life. These would be things for you to look into. Perhaps he is still growing up emotionally and you're showing him how to do that.