A few things to be aware of here:
Mother may have condition(s) that rule her out from donating, hence no test.
The matching process, if the DH goes that far, will reveal whether or not he's a close relative of this boy. There's a box you tick to acknowledge this, because it can be a very difficult moment if people find out they aren't related.
People create a lot of drama online and in life with the idea that a close relative is needed for a match. They aren't, and close relatives often don't match. But here's the thing:
you don't need to be a match to donate a kidney. Say you are A and want to donate to B. You don't match. You go into a database which runs four times a year. You are paired with C, who wants to donate to D. You donate to D. C donates to B. Any adult who has been cleared healthy enough to donate can do this. So the risk of running out of matching donors for future children etc isn't huge - but if you're being told uncles didn't match as opposed to didn't qualify as healthy enough, that's misleading.
There may be no immediate urgency and in any case, DH would have to have tests of kidney function etc. Once you are matched or paired, you can choose preferred dates.
All operations under GA have some small risk. If your DH has above normal blood pressure, his risk of dying is a little less than it would be for a woman giving birth. If his blood pressure is normal, it's far far lower. And it's getting safer all the time.
Morally I would say a parent should try to donate if in reasonable health - I wouldn't say this of any other relationship. I've done it. It's fine - like a Caesarean mostly.