Try a "play and leave" strategy. Sit down and set up a game with him; train track, zoo or whatever. Start an imaginary game with what you've set up (try to prompt his imagination rather than him relying on yours). Once he's into it, leave the room. Pop back in every now and then. If you sit down and play a game like that every day, over the course of a few weeks the time you need to be involved in it will lessen.
When he's tugging at you wanting your attention, what do you do? If you are doing something essential then you need to give him a time when you can play with him ("I'm doing some housework, I'll be finished in ten minutes, if you can be quiet until then we'll have a lovely game.") If you are relaxing, then its best to (before you sit down and put the tv on or pick up a novel), play with him first, but with a "deal" that afterwards he continues the game on his own.
How much screen time is he having? If its a lot, that will affect his attention span, a problem for you at home and for his teachers when he starts school. Try to limit screentime, so he has to choose whether he wants computer or tv, and he helps add up how much he's had each day. My DC age 6, 4 2 are only allowed half an hour in termtime weekdays, 3 hours total over a weekend, and an hour weekdays in school holidays. Some days should be completely screen free. Also, their screentime is a reward - bad behaviour has some knocked off. He will only accept these boundaries if you model the same behaviour. I don't watch any daytime tv, and limit my daytime pc use strictly; there is plenty of time after they're asleep.
So have a plan for most days of the amount of screentime, one or 2 "play and leave" sessions, a storybook time when you read to him and then discuss the books, and get him to join in with age-appropriate jobs you're doing, such as making the lunch, sweeping the floor, etc...
You sound like a very loving mum; we expect all these things to come naturally to us but actually we need a bit of strategy, ime, and its great that you've realized that and have asked for help. It gets much easier when they start school! But having more of a routine and structure for daytime at home will help him with the structure of Reception.