OK, Herby, here's the deal. I'm old and very crabby (natural consequence of getting older). I am guessing that you are young. So maybe you might be interested in some advice from an old crab.
As you get older, one of the biggest regrets you can have is not having done right by a dog. So you really want to avoid that.
I'm guessing here that money might be an issue with getting another Boxer right away for your Mal. Understandable that you would not want to post that as a reason - if that's what it is - on a public forum.
But a good quality Boxer doesn't have to cost a mint.
Your best bet might be a retired breeding Boxer female. These are often not very expensive - the breeder is mainly interested in a finding a good pet home. You can call some Boxer breeders, explain your situation, and ask if they have anything like that available or if they know someone who does. You would want to aim for a Boxer on the younger side, given the shorter life expectancy of Boxers. But some Boxer females are retired young from breeding because of a delivery catastrophe that resulted in a C-section and spay.
You can often get a really nice Boxer pup at a reasonable price from a family with a pair of nice Boxer family pets that they occasionally breed. Just make sure they've done the appropriate health testing. Places to look;
Just make sure you meet the parents and that you're getting a Boxer and not a Pitbull or Pitbull mix.
Unfortunately, I cannot recommend getting a Boxer from a rescue group, for two reasons. First, many rescue dogs have behavior problems (like dog aggression or worse) which you and your Mal do not need to deal with. Second, a lot of rescue groups pawn off Pitbulls as Boxers or Boxer mixes. And you are not looking for a Pitbull.
Many years ago, I made the mistake of getting a rescue male Whippet for my grieving female Whippet. He immediately attacked her. Back he went to the rescue group - which was furious that I didn't keep him - and I quickly found a retired show male Whippet who was perfect for her.
I strongly recommend that you get a sweet, good-natured, cheerful female Boxer. It's great that your two males were such excellent buddies, but male-female pairs are the safest bet.
Finding a reasonably priced Boxer like this will be a lot less trouble than hiking all over the place with your Mal trying to cheer him up. And it will make him a lot happier to have a Boxer babe.