I'd guess dyscalculia, as well.
Also ADHD here (as is my son), and we skip letters, skip numbers, skip chunks of sentences, combine 2 words into one (although they usually make sense... the words we're LOOKING for is a combo of the two words we mash together), create new words... but the REVERSAL of letters or numbers tends to be a co-occuring disorder.
I'm MILDLY dyslexic in addition to being ADHD. It was diagnosed on accident when I was in the 6th grade (I'm a fast reader, I just went off of context), but it was HIGHLY annoying... so I taught myself to write backwards. Like sdrawkcab AND (the keys won't let me do it) mirror image backwards AND I copy other people's handwriting (I'm a fairly decent forger at this point). I was on a playdate with a dyslexic friend and her therapist "caught" what I was doing (copying my friend's handwriting and writing it mirror image), and did quick testing. By the time she'd gotten to me, I'd self corrected most of my stuff (my brain sees numbers and letters both forwards and backward and upside down)...but she got me into therapy with my friend. I was out of therapy in less than a quarter (both because I only have a mild case, and because I'd already spent a couple years training my brain to see letters in any direction. But I still see/write bob dod pop and end up scratching things out. God I love keyboards.).
WISH I'd gotten into therapy earlier, it made things 100 times simpler, but then I might never have learned to read and write backwards, mirror imaged, and upside down. The forgery has never been useful, but it's a cool party trick.