What is the difficulty he's having with fingerspelling??? I'm dyslexic, and I can't imagine much, seriously!
The issues with spelling that dyslexics face are related to written text and the reversals/transposing of letters on the page. When fingerspelling, the letters don't change from one to another depending on reversal (ie left vs right handed) or anything. Unlike written letters for example b d p q n u etc. I think he probably just needs more practice and is blaming the dyslexia, everyone struggles with some things sometimes this is just something he might need to work on more. The only way I can imagine dyslexia could be impacting on his fingerspelling is if he hasn't yet had help learning to overcome the basic early-childhood type effects and his literacy skills are so poor so he can't spell/read much in general.... but there are plenty of Deafies whose spelling isn't their strongest point and they still communicate fine by sign ofc!
Regardless, he needs to talk to the school dissability support person about accomodations, not the teacher. It's fine if he just wanted to make you aware that he has the issue so you understand where he's at, but if he wants specific help, accodations etc, he needs to supply the dissability support people with the appropriate doccuments etc and work with them on this one. He shouldn't be graded differently, but things like extra time on tests, help with reading material etc should be put in place so he can learn the material like everyone else and earn a regular grade.
I'd encourage him to keep trying, to work on his spelling in general, and to talk to the dissability support people at the school!