German Shorthaired Pointer Q&A
A wire slicker brush and a fine-tooth metal comb. First brush, then comb. He won't like it if he's not used to it. Mine gets treats for putting up with the process.
I just cut him down, they hair grows so fast less then 6 weeks. If you can't cut your down, slow brushing and oatmeal conditioner
If clippers you could use a 7F blade for matte stripping or use thinning scissors.
I would take to groomer, they know proper way to deal with matting without hurting the pup.