@hlei7 Shoulder pain sucks! I am so sorry your husband is faced with having a shoulder replacement. Each person's journey through joint replacement is different and with care and careful rehabilitation your husband MIGHT be able to return to his career with some modifications in how he does things. When I had my first shoulder replacement (right = dominant side) my surgeon told me I had a 20 lb weight restriction on how much I could lift. But a couple of year later when I saw him about the other shoulder he said "no, you don't need to keep to that restriction". I'm a LOL (little old lady) so I don't regularly lift heavy things ... but I can if I need to.
Are both of your husband's shoulders paining him? If only one is being problematic presumably he can still do heavy lifting with the "good" shoulder. As for up and down motions for painting ... replacing the shoulder joint might shorten his reach a bit but would not (IMO) negate his ability to do that. Recuperation from shoulder replacement can take 4-6+ months and should be done slowly and allow healing first before doing any reconditioning and strengthening.
It might be in your husband's best interest to get a 2nd opinion from another shoulder surgeon - one who is not in practice with or associated with the first surgeon.
Best of luck to him and do ask any other questions you and he have. We do like to help.