He was dyslexic, a fact well known today – he’s become a figurehead for proving dyslexic children are not unintelligent. Today, ideally, he would most likely have recieved early intervention which meant his early writing skills would have been improved and he would not have been considered globally slow because of one impairment. This would not have affected his IQ or ability to create his theories either way, but might have made his childhood schooling easier or less upsetting. His ability to come up with unique thoughts and theories and reasonings appears unaffected by his environment and schooling, though this is based on anecdotal evidence.
(Some schools lack the funding for these programs, or the staff, so I say “ideally” above)