Firstly, if you are not doing it already, read to him every night. Go to the library and let him pick some books. Make it a comfy time. You might want to introduce hot chocolate or something else as a treat with it too. Aim for enjoying books together in a relaxed environment with no pressure for him to read. Don't underestimate the importance of this. He is still gaining vocabulary and hearing a variety of sentence structures and learning comprehension skills like building a visual image from the words. It is very good if his Dad or other significant male can do this some of the time too.
Secondly, make sure he sees his dad or other significant male role models actually reading a variety of things. If helps for boys to have make role models.
Then introduce a routine for the holidays. I'm not sure whether you are working or not, but if possible introduce 10 minutes of reading practice as early in the day as you can. You can use his school reading book if he has one and ask advice at the library for books at a similar level to reading books he has been on .
Say to him that you understand he needs screen time to unwind but other things are also important so make his screen time conditional on doing the reading. Eg, 10 mins reading equals 1 hour screen time later. Make it so that you can add in additional tasks for extra time later
Explain to him that you are concerned about the way he behaves when he has been told to come off a screen. It is your job to help him with managing life and the logical conclusion for his behaviour is that if he can't manage to come off screens then he should not go on them.
Then get him to practice, 15 mins on a screen, you give him a 5 minute warning and then he come off without a fuss. When he can manage 15 mins you can try 20 or 30. Not managing to come off without a fuss means no more screens that day but you can practice again tomorrow. Don't be angry or punitive in tone if he has a meltdown. Be sympathetic "I can see that was tricky for you. We can practice again tomorrow "
Out all of this into some clear timetable so that he knows when he has to read, when he is allowed to go on the computer and for how long, for each day of the week.
Once you have this structure in place then you could add in another 10 minutes a day of maybe a maths app or revision book or whatever ( he may benefit from a year 3 one to start but take off any covers with references to year/age) in order to rear another hour screen time.
It is likely he will resist some of this to start with but use times when he is calm to explain that you are doing this because you care for him and don't want him to find next school year too tricky and that it is not healthy for him to be on screens all the time and that it is your job to make sure he does healthy things, just like you make sure he has healthy meals etc. Then try and calmly persevere with the routine.