I often think that people are too quick to label inabilities with a disability (dyslexia, ADHD, autism etc). At the end of the day, some children - and adults of course - are just well, without being rude to you or anyone, a bit ... thick. Or at least not as 'intellectually tuned' as those of similar age/experience.
As I said, if you are really concerned - he is only 7 after all - give the coloured sheets a go and if it seems to improve his reading (although this may be just because the coloured sheets adds some greater interest in the reading material!) then it may well be mild dyslexia. Good luck.