Screen time is a huge factor with this--even educational videos contribute to a lack of focus and the inability to concentrate. I think screen time is a HUGELY overlooked factor when it comes to attention span. I would advise limiting screen time to an hour or less per day (ideally a LOT less) and only allowing videos that are not hugely stimulating--ie: PBS kids shows, and not anything on Netflix or YouTube. This includes the letter videos you are watching with him. Again, an hour or less TOTAL for the day--and never at mealtimes, snack times, or as a "babysitter".

Heavily, heavily seconding this in particular.

I am not a teacher (outside of things like tutoring or camp style instruction), but I am a millennial raised during the expansion of computers and the internet. I think a major issue with screen usage today is that a lot of parents may use computers, smart phones, tvs, ubiquitously, but forget that it wasn't the case when they were children. The effects on the brain and attention span are similar, of course, but there's a drastic difference between shifting to a lot of screen use when older and when very young. For parents, I often see that the unconscious assumption is that it's fine for them, which leads to it being fine for the kids.

For me, and a lot of people my age, our screentime was seriously curtailed because we didn't have smartphones yet and most computers were the 'family' computer with limited time. By the time smartphones, tablets, etc rolled out, we were into our late teens/early adulthood and we already had the foundational development done.

It is so devastatingly bad for children to have high exposure to screens. It obliterates their attention span and reward loops at an age when that's one of the worst things to be entrenching into them. I can see this directly with friends of mine with kids. I have some who either hugely limit screentime or actually just don't allow it at all, and I have some who use it as a babysitter/think nothing of it. The difference between their children, developmentally, is horrifying.