The OP's husband is tall, not overweight. He needs legroom, not rear end room! Buying an adjacent seat isn't going to help. Southwest's customer of size policy doesn't apply to him anyway; it applies to those who can't fit in a 17 inch wide seat.
I would suggest in the future booking an airline which offers a buy up to extra legroom seats, especially for longer flights.
Size is size. Being 6'8" means not fitting in a seat with 31" pitch, so personally I'd argue that he should be able to take advantage of the Customer of Size purchase option.
Having an empty seat next to you means that if you raise the armrest you can sit with your legs at an angle, which allows much more legroom, an extra 13 or so inches of it.
Of course, on SWA planes, tall people who are in the know covet the starboard exit row window spot behind the so-called
LUVSeat. There is no seat in front of it at all, so the pitch is an incredibly generous 62". (On older models of the 737 it does not have a tray table, but that's the tradeoff.)
However, as we've noted before, the OP's DH won't be eligible to sit in the exit row unless he arranges in advance to temporarily disown his family. I will note that on one or two occasions I have seen the GA give a really tall man a blue sleeve so that he can preboard to try to snag this seat; normally blue sleeve preboards are not allowed in the exit rows at all, but this is an exception. However, you have to be REALLY tall to have any shot at obtaining this special privilege; I'd say at least 6'5" at a minimum.
PS: The business about the armrest is primarily a test for a Customer of Size who doesn't want to be one; it is the test for NOT being required to buy the second seat. AFAIK there isn't a test to disqualify those who want to voluntarily purchase it because they don't want to encroach on another passenger's space.