Carbs are much less of a problem for T1 than T2 spectrum, so carbs are not really an issue (I can have 60g starchy carbs in form of wholemeal pasta without BG going above 6.5); having said that, there's a whole range of Humulin products and if he's on twice daily then it's probably a mix. If it is, then he will have to eat a certain amount of carbs each meal and possibly snacks to avoid hypos, and I would definitely stick to the HCP's advice.
With more flexible regimes (basal bolus, usually Levrmir/Lantus and Humalog/novorapid/apidra, or pump) it's possible to have more flexible meals, including very low carb. I'd keep ketones nil though (i.e. total carbs > 50g or whatever it is), if only to be able to reliably tell DKA from miscounted carbs.