Son got cheated on and I don't know how to console him

So my son is 17, his girlfriend is 18. They've been dating for two years. In the grocery store last week, an older boy (college aged) greeted him as if they were familiar with one another. My son was confused but greeted him back. The guy was talking about how Angela never let him meet her family and that it was coincidental he bumped into him. My son asked what he was talking about and the boy said you're Angela's cousin I saw a picture of you on her phone. My son asked what Angela are you talking about because he was getting nervous and the guy said her full name, confirming that was my son's girlfriend. My son got mad and told the guy to walk outside with him. I just stayed behind in the aisle. When my son came back he was in tears and angrily throwing the stuff he wanted in the cart. I asked him what happened and he told me that he called Angela in front of the guy to prove that they were dating. Then he told her "Someone wants to speak you" and she said who, he gave the phone to the other guy so he could talk. She started crying and apologizing to them both but they basically told her to fuck off. It's been a week and my son barely wants to do anything. He only eats once a day at most and he'll just lay in bed on his phone. When I ask him if he wants to talk he asks me to leave him alone so I respect his wishes. I just don't know what to do. It's especially bad because he pays for everything she can't afford. He's been paying for her birth control ever since they became "active," he helps her pay for her insulin because she has no insurance, and he paid $400 for her dog's gastropexy. He works a part time job so understand that all of this is literally his entire paycheck. Whenever I asked him if he had money he'd only have like $2 in his account at most. I just don't understand why she did this. This family pretty much adored that girl. Sadly I have a strong feeling he will forgive her because he's so head over heels for him. It's his choice if he does but I hope he's smarter than that.