The W-4 is used to estimate a person's tax situation so the right amount can be withheld. It is not binding and has no legal force. Your boyfriend must file his taxes legally and correctly no matter what his W-4 says. If he is unmarried and has no dependents, then he must file as single.
If his W-4 was withheld as HOH, he may be under-withheld, meaning he will owe additional taxes with his return. To prevent this next year, change his withholding to single (unless you plan to get married or have a child).
