I agree I wouldn't rule out vision. Get it tested by a specialist. It never hurts to get the obvious checked out. There are many different types of vision issues. Just because he CAN read things when he's concentrating doesn't mean he's not starting to have vision problems. My understanding is vision issues often come along slowly, get more prevalent as kids get older. And that the start of vision problems don't make reading impossible, just difficult.
What I find is odd is that he can read words well but he isn't just reading everything is site automatically. In comparison my son at 7 didn't like to read novels/fiction. But he taught himself to read at 3 and by 4 couldn't help but read every and all text he saw. At 4 going to a museum/aquarium/zoo he would read each sign out loud and in order. He quickly reach a point where he couldn't help but read.
Keep in mind there are a lot more reading issues than reading than dyslexia. My older DD had language processing issues. While she could read individual words just fine, she had difficulty stringing those individual words together into a sentence with meaning. She looked like an on target reader in K & early 1st grader but when other kids took off with their reading she stagnated. Doesn't sound like this fits your son but there are many different types of language & reading disorders.
Also one thing to consider is he is a 7 year old boy. Gifted kids can be very asynchronous. And average 7 year old doesn't read really fluently at this stage. That although he can decode words just fine perhaps he is just taking a bit longer on the other aspects of reading, that it just hasn't become automatic enough. There is a lot more for the brain to put together than just decoding individual words. It's also possible that HIS reading is too slow for him. It might frustrate him because it takes too long & he misses the meaning while struggling with decoding. This is why I advocate that even with the most advanced reader, you don't stop reading TO your child. Perhaps expecting him to concentrate at a museum/zoo/etc. is putting him on the spot and asking him to slow down too much. At 7 is might be better just to read the signs to him. I read nightly to both my kids till they were much older than 7. Picking books that were what I though of as beyond their reading level. I have no idea what your parenting style is, but I see many parents who stop reading with their child once they figure they can read. Encourage reading by sharing it with your child.
On the other hand I'm wondering if it could be a more modern age video/tablets/computers are so prevalent issue. It's so easy to find video's on almost any topic online. Why should he bother to read for content instead. It easy to entertain oneself with video games & movies. Educate oneself with video's. Why should he bother?
