The professional groomers would probably be a better resource, but I can give a few tips. My Maltese was really matted/dirty when I adopted him. I didn't want to put him through the stress of going to a grooming shop, so I DIY'd it. I put him in the tub and doused him with conditioner. I think I used suave, but any thin hair conditioner would work. W/o using any water, I slowly worked out most of the mats. Bailey was very patient about it and I wouldn't have done it if he had seemed stressed. After all the mats were out, I washed and rinsed him really well. Unfortunately his hair was so damaged (yellow and brittle) that I ended up clipping him anyway. Being mat free and clean made the clipping go much faster, though. Do you have good clippers? You can try clipping under the mats but it's harder and the dirt is damaging to the blades. If you're using petclippers (they don't have removable blades) this is really bad. Can they groom him at the vets? Or maybe you could have a housecall groomer come to your home? (The latter may still be a concern b/c he's infectious.) Good luck with him!