Is he taking methadone under medical supervision? Presumably his doctor would have let him know what the risks/benefits/possible side-effects of this change would be.
If he's decided on his own that he's going to take less--no, that's not smart but again that's his choice.
In either case, if he finds the dosage change has negative effects it'll be up to him to respond appropriately by going back to the doctor or whatever.
As far as pointers for you-- and I don't mean this at all in a glib way-- just keep on doing your own thing, having your boundaries. The recovery is his deal altogether.