When signing up through healthcare.gov, he can give his income as a yearly estimate. If it's significantly lower than what the system has on file for him, he will get a message to the tune of:
"hey looks like you usually earn more than that, what's up"
and there is an option to say
"yeah my situation is changing, trust me on this".
Have him go through the signup process, there's no obligation to actually pick a plan.