Do you have a quarantine tank, or can you set up a temporary one for him?
A lower water level can help a fish with swim bladder issues. It's tiring and stressful for them to keep trying to remain upright and near the surface when there is a higher water line, so having a reduced water level can make that easier for them to recover while you figure out the cause of the issue and treat it.
Of course, it's crucial that the remaining water has excellent water parameters, with 0 ammonia or nitrites, and very low nitrates. Can you give some more information please? We cannot help much without knowing your water test results, stocking, what you feed, tank size etc. There are a few different things that can lead to swim bladder problems, some of which they can recover from. Have a five year old black molly who had a pretty bad case of it, left her swimming with her head pointing up and her body vertical at one point, poor thing. But with treatment and time, she recovered and has been fine for years since then.