For context, he did something pretty bad (I don’t want to go further into it) that made him go away. In our last phone call he said he probably couldn’t talk to me for a while and I was so upset with him (for other reasons).. Then I tried to call him just to check up and I find his number is gone, changed. Should I just leave him alone? This happened 5 days ago and I don’t know if it’d be appropriate or not to send a letter to his house because his parents told him they don’t want him speaking to anyone online. We’ve been dating for 5 months and his parents know about me but I’ve never spoken to them. I am 18 and he is 20.
Do you mean he did something bad to you, or plainly messed up irl? Since his parents know of you, yet disallowed online contact, I guess the ban is related somehow?
I'm not quite following on whether you've met in person yet, but generally for an online relationship, you cannot and should not try to force contact beyond the means agreed upon (eg going beyond an occasional reminder), as it's unlikely to bear a positive effect, much like recklessly crossing boundaries in any context, with a greater extent of privacy assumed to begin with. Dissolution is common for the lack of tether, but it's not like relationships come with warranties for happiness on a whole.
So what I'm saying is, wait a while. Once he decides he's ready he knows how to contact you. If not, forcing him into a corner won't change the outcome.