I agree with asking his therapists or the doctor’s nurses about places to shop.
There are also gadgets online for help with things like putting on socks, but they can take some practice and may not work for everyone.
Here’s a link for one from Amazon.
If he’s still seeing Occupational Therapy, they could try some things out with him - since he’s having issues with dressing, he might be able to get a referral from his doctor for some OT sessions to work on ADLs (Activities of Daily Living).
For shoes, some cities/areas have shoe stores that specialize in fitting shoes for special situations - therapists would know if/who in your area.
Billy Footwear has really innovative shoes that are very easy to put on.
They have laces for making them fit closely, but the front side zips open. Unzip them, place the foot in and zip it back up.
Skechers makes slip on shoes and some slip on shoes that are ‘hands free’. A lot of stores carry Skechers, so you might find those styles in Macy’s or Kohls.
Adaptive clothing tends to be more expensive, even though it is moving into Mainstream stores like Kohls and Target; there is just not a very big market buying them. If you do look at adaptive clothing, look at whether the features will be helpful or not. A lot have adaptations for difficulty using the hands - for example, Velcro in place of snaps, buttons or zippers. Some have adaptations for access to things like feeding tubes or stomas and some are designed for people who need to dress lying down - for example, pants with Velcro on the side seams that allow the pants to open flat. So, the adaptations they have might not be at all helpful.
An OT could help you figure out what might be helpful, but you can figure out a lot by yourselves. He says the pants are too tight - what part is too tight?
If it’s getting the leg opening over his foot, boot cut pants might help or you might be able to find a seamstress to open the seam with zippers or Velcro from the bottom up about 10-12 inches to make more space. If they just seem too tight all over, pants Relaxed fit, pants with more stretch, a larger size or jogging pants might work. If he wants to shop at Macy’s, I don’t know if they still have personal shoppers. If they do, that might be a way to narrow down the selection if you can tell them what features he needs.