We chose an HM who was doing the job when we met him, over a really nice one who I think would have started at the same time as my DS would, anyway he certainly wasn't doing the job when we met him.
But at DS's the HMs have loads of autonomy, he interviews prospective boys and decides whether or not they get a place in his house and the school in general, every house is different and he is the most important person in your DS's school life so seeing how he does the job is kind of key.
I worked closely with a new HM (different school) the general consensus is that it takes them a year to settle into the job, I think it's hard to walk the fine line between formal/informal, it can be tough on their families, the hours are very long and they need to balance this with their own family commitments, they are often dealing with very demanding parents, and often older pupils (and even parents of older pupils) resent the change however good the new incumbent is. It often unsettles existing pupils who have got used to the way Mr X does things. Existing house staff can also find the change difficult. So the first year can be a bit of a shock for everyone. On the other hand a new HM can bring about a much needed change, it's inevitable as the current encumbent is coming to an end he's less likely to address any long term problems. After a significant stint in the job most must be completely exhausted and their own families fed up to the back teeth with the job.
In your position I'd meet with him, don't ask too many questions let him do the talking, generally having excellent interpersonal skills is one of the requirements of the job although Id want to ask him about previous experience of boarding and where. Are you a good judge of personality? Some people aren't, my friend I walk my dogs with is absolutely crap Im always amazed because she's generally a very capable person. On the other hand I have spent 30+ years mainly on the front line in public sector (I'm a cynical old bitch and I can detect a nutter at 50 paces) and I generally considered a pretty shrewd judge of personality. If your good go with your gut feeling. Be friendly and pleasant, don't talk to much, listen to what he says, try and find out what his ethos on life in general is, I'm assuming your taking your DS watch how he behaves around him. He will be loco parentis, it's great if they amusing, urbane and charming but your not looking for an dinner companion, he should IMO more importantly know his mind, be decisive and have an air of calm authority about him, he's going to need it running a boarding house. You should feel he genuinely cares about your DS. He's acting on your behalf standing in for you on a day to day basis and will be an important part in your DS's life how do you feel about that? Do you feel confident in him?
If you come away with any significant doubts ask to meet another one so at the very least you've got something to compare him against.