Why are you shutting down his ability?
Yr r is just an extension of nursery. It's for playing and learning to get along with your peers. If he's struggling with that, then school will help.
Take him to air shows -he'd love the harrier demos if they still do them.
Lots of kids are v bright, but they still enjoy kids stuff too. Jumping in puddles, going on the swings. Walking in the woods and poking dirt with sticks.
All of those things don't preclude an early interest in electronics or being above average academically, and experience of those and learning to role play will help him bond more easily with his peer group.
Personally I kind of leave it until yr 1 before I get too concerned about schooling, but you can discuss differentiation at any parents evening.
(for perspective - dd2 was assessed at between 5 and 7 years ahead in yr r. It was all fine. Generally kids move through school with their correct peer group, although v occasionally kids move up a year - it can lead to further social exclusion. There are loads of threads about it on here)
Loads of people do kumon, but it is very rote learning and doesn't really encourage thinking and problem solving. It depends what you want. It won't harm him. Some areas also have explore learning as well.
As he gets older, it's generally assumed to be better to go sideways rather than getting too far ahead of the peer groups curriculum (it's also reasonably common for kids with high ability in particular subjects to be moved up a grade or two for those, so I would look at those as 'peer group' as well, even though they aren't iykwim) does he play any instruments? Music and learning to read music is a good extension at any age, and mathsy kids find that learning an instrument alongside is beneficial for their maths too!
He does sound like a typical little nerd though, lol. 
there are loads of kits that would enable him to explore the electronics and building side more. Lego technics (or whatever it's called) loads of electronics sets where you get to build radios, burglar alarms, flashing lights etc etc. all things he can be playing with and learning about how electricity works as well. The train thing sounds fine, but it's just really putting a track together and using a controller, right? He doesn't have to actually build the circuit boards?
Honestly, as long as they have assessed his reading, I wouldn't stress about school academics until he is into KS1. You can help him have fun at home with all the stuff he is interested in. He'll keep learning anyway. No-one learns because a teacher tells them something. let school carry on with helping him get on with his peers. That will stand him in much better stead at this point than algebra. Plenty of time for that in y1.
Is he an only child btw? Sometimes they do appear v adult as a consequence of not being around other children/siblings v much. If not, he obv just has that little professor thing going on. And if he does, the teachers will notice and react accordingly.
When is your parents' evening? Just ask how he is doing in relation to his peers. Ask about differentiation, and ask what his weaknesses are and how you can work on them at home. (always good to suck up a bit)