I can tell you that my 18 yr old son will not answer a phone or text if he even gets an inkling that his gf or girl in general is being in any way over-dramatic etc. He is just that way, always has been. He's broken up with girls for not leaving him alone, or coming over when he asked them not to. So I can see HF not answering texts or phone calls especially if they've fought this way in the past. He must figure she is going to get over it and show up sooner or later.
IDK if HF is involved I kind of think its unlikely, I also don't know your sons situation, but the behavior in your son is common. HOWEVER when you live together and it has been four days and the last communication was I'm coming home leave the door open ... possibly from some other dudes phone .... then I think things are a little different.