I hope that conversation was a positive step forwards.
I am rather world weary and would be careful not to fall into a dynamic where DH runs from one to another placating and being torn yet trying to be 'fair'. As that kind of behaviour doesn't deal with the issue and sets up an either/ or type of mentality, and the fairer person never wins as they get dragged into having to be understanding at the expense of their own needs, and the person behaving worse somehow gets a 'get out of jail free' card. Ring a bell so far?
I say this as I've seen it before, not because I know it will happen. More of a forewarned is forearmed kind of thing.
It's not fair that DH has slipped into being the parent for his brother, and longer term it would be healhier for him to develop a more equal type of relationship, but quite frankly, that's a massive journey which only your dh can decide to go on, and is fighting entrenched dynamics fuelled by guilt and fear and love.
I'd suggest steering well clear of that particular area, and concentrate on getting DH to see how his reactions are not actually helping his brother even though it comes from a place of love and caring...
I would concentrate on getting DH to see that his behaviour is NOT protecting or helping his brother, and as he's taken on the role of protector and nurturer, he might as well do it properly!
By showing his brother that the normal rules don't apply to him, and that he's not expected to behave in the same way any grown person does (or any growing person whatever their age, actually!) ... all that's incredibly damaging for the brother.
Does dh want to show his brother that he's not good enough to be treated as a normal person? Or that dh believes that his brother isn't capable of reaching even the bare minimum of nornal behaviour and interactions? As that is the message he's actually giving his brother, whether he means to or not, and how is that in any way a positive, nurturing thing to do??? It's limiting, and keeps the brother in a vulnerable, child like position, with more reasons to continue being a stunted immature and manipulative whiner who gets to be the centre of attention for all the wrong reasons.
How would dh behave to a child behaving badly to get what he wants? Trying to push someone else out of the family as he's jealous and insecure? Like the classic brother osje alpha of new baby/ family member? In that scenario, would your dh see his role as helping the jealous child to come to terms with his emotions, show him how to channel them into more positive behaviour, show him that he is still loved and valued, in conjunction with setting firm boundaries and rules, praise for good behaviour and refuse to 'feed' bad behaviour?
Or would he encourage the bad behaviour and collude in pushing that other sibling/ family member out? Would he reward bad behaviour by lavishing love and praise on the child each time they acted out? Would he let the child believe the other family member wasn't as important as himself? And that he has the god given right to come first in everything, and that it's fine to treat the other family meet like a second class citizen? Would he make excuses and tell everyone to tiptoe around them and pretend not to see when the jealous child hits the other child, or pushes them over, breaks their toys, or shouts rude words and he really starts a campaign of bullying and nasty behaviour designed to keep himself hogging all the attention, time, effort and deliberating trying to deny the same to the other child, who's none nothing wrong except existing?
Because one is being a responsible adult teaching and helping a child develop his potential, instill a sense of right & wrong, help build a positive self worth and good mental health, and help the child grow and mature and develop into a great person. Or of course, he could be the type of person who just let the kid have what he wants (rather than what he needs), call the shots, and panders to destructive behaviours and generally helps create a monster?!
I know what I'd do if my 5 yr old was behaving that way - and it seems already I expect more of my little boy than he does from his grown brother! And if he's the one taking on the 'parental role', then he should take responsibility for the consequences of his dysfunctional 'parenting' / the lessons his brother learns from how dh responds.
By the way, it's obviously not ideal that your dh has taken on the mantle of parent to his brother, but that's a harder nut to crack and you can at least start from that reality and get him being more effective in that role than the dysfunctional stuff he's locked into now albeit with the best of motives.