Fit is realtive to how his bio mechanics are. If he aligns naturally over the skate blade he can basically get away with skating in anything, but with performance degradation as the fit gets worse. However if he does not align naturally over the skate blade, eg pronation or supination and or other issue/s, then even the best fitting boot only hides the problem. Every Haglunds I've seen in skating comes with associated foot rotation in the skate because of bio mechanical issues. A good fitting boot will help but I'd also be looking at his overall bio mechanics: gait. posture, balance, leg alignment etc and then getting the skate adjusted to suit eg shimming, holder alignement, foot support. A good fitter, typically from the figure skating side, should be able to help here or at least guide you to someone who can help. For his long term skating health you want to be trying to address the cause.
And since he has a haglunds bump, whatever skates he gets you want to make sure you get the boot punched to accomodate it. I wish you the best finding someone who can help, unfortunately I can't provide any names but if you want him to see someone outside of skating who has experience with foot issues / skating then have a read of this: https://www.aapsm.org/pdf/humble-skatinga.pdf
and maybe find someone near your location from here: http://www.aapsm.org/members-south.html
Or he can correct any bio mechanical issues himself if he has the opportunity to put the time into training AND has the dedication to do so. pm me if you want more info.