I'm sorry about the loss of sleep! It's a pain, I know.
First, please be aware (and be sure your husband is aware) that this is NOT an issue for discipline -- your child is not being bad or naughty on purpose to get your goat. Kids' sleep patterns change their whole lives, so a child who slept perfectly all night will indeed have periods of waking. Even my child at age 12 has changes to her sleep patterns.
However, though your son is not intentionally "misbehaving" (so please don't treat it as if he is and ensure dad's on the same page), he is indeed getting plenty of attention in the night, and that has two results: He will keep seeking that attention, and he will find it ever harder to get back to sleep afterward.
You mention that you've tried a lot of things -- getting on the couch, giving in to his demands for drinks, etc. It's time to find just one single response that both you and your husband use, the same way, every single night.
First, this has to be boring for him. He gets up, you walk him back to his bed, saying nothing. Do not interact with him when you take him back to bed--no scolding, no pleading, nothing. Silence. You may have to return him to bed over and over and over but DO it because not doing it sends the signal that it's OK for him to be out of bed. And do it silently because any talk -- and especially letting him have what surely is a treat to him, lying on the couch with you -- just gives him attention for getting out of bed; basically in his mind there are rewards when he gets out of bed. You want him to learn (and it takes time and more sleepless nights for you) that when he gets up there is no payoff, no attention, no interaction, certainly no couch time or drink. Consistency and handling it the same boring way every single time is critical.
Checking him for a fever each time he wakes is frankly an overreaction; if you continue that as he gets older, he's going to think he is indeed sick or there's something wrong with him. And again, that fever check, plus the yelling for a drink and the couch cuddles etc., are all letting him spend way too much time awake - he gets more stimulated with every minute he's being attended to and needs for things to stay dark and quiet and he should remain in, or be escorted silently back to, his own bed.
You mention that "he rarely takes a nap unless he's grouchy or tired and falls asleep himself" and then you wake him -- He doesn't have a regular nap with a naptime routine? That could be a very big reason he's up at night, even though you feel that less sleep in daytime interferes with nighttime. An over-tired child is as bad as one who has slept too much all day, and he may be so over-tired that he gets wired--truly, a child can be so tired that he then cannnot really sleep.
Please try to establish a daily nap for him, and that may mean a full routine like bedtime but without a bath. If he has been taking naps up until recently, then the issue may be that he is starting to drop his nap instead (my daughter stopped napping at two but we encouraged her to keep it up until three). If that is the case (he has been a napper but is giving it up gradually) then do encourage a quiet time each day so he can at least rest and (hopefully) nap without having to get so tired he's grouchy first.