Grooming older Terv (thinning scissors)

I have a 13 year old Male Belgian Terv. Between the top coat and the undercoat, he has a lot of hair. My problem is that because of arthritis and general pain he is very sore. To brush him makes it worse and forget the groomer - they are too rough despite what i tell them. So I have to brush him myself which isnt a big deal but he gets sore from my gentle brushing. Plus because of the soreness I admittedly ignore grooming. The big problem area is the flank to the hind end. His undercoat is huge. His britches are long.

I was wondering about using thinning scissors. Would this help me get to the undercoat? I hate to cut the topcoat in the hind end (I really dont care what it looks like at this point). the Belgian Terv people would die. I have to get to all the dead skin and loose hair though.

This summer I used the deshedding shampoo which made everything easier. It is winter though.

I cant take him to a hand doggie wash or use the bathtub because of his huge anxiety. He has had a couple of seizures over the last 2 years and the last one was brought on by taking him to get his nails done.

If I were to make a mistake this is the time of year to do it because he will be losing his entire coat.

Thanks for any suggestions.