It's such a hard time, the is he/isn't he phase - I am sorry.
Have you looked at any books/websites? There is lots of info around about how to treat him, how to help him, and how to help teach him.
Wrt to how to approach and deal with this specific behaviour, I would actually persevere with encouraging him to expand his play and activities. One of the things common in asd is repetitive behaviour, and ime left unchallenged these could become more and more ingrained (I certainly saw this with my own ds at that age, before we started any intervention).
You could try simple techniques of giving him massive rewards (reinforcers) when he does something new, tries a new game, accepts it when you switch off Abney & Teal etc. It's kind of basic behavioural intervention: if you increase the reward for something, he'll associate that thing with something good. The harder it is to drag him away from A&T or whatever, the bigger the reward, which will increase his motivation to do that new thing again. So a reward for something which he does willingly could be something like an extra tickle, while a reward for something which is harder for him (like stopping watching A&T) would be much bigger, maybe a small food item. Even just small things like starting a game of eg musical statues (or something you think he'd like) and he joins in instead of running round spinning his arms would get a reward.
Not everyone favours this approach (we've done ABA for 3 years), but we found that the longer we left ds to his own devices, the more narrow and rigid his world became. We were pretty sure he would not just 'grow out of it' and just start doing new things off his own back without any intervention, and just couldn't bear to see him getting increasingly rigid and repetitive about things.
Our general rule is basically we try and intervene with anything we think interferes with him functioning, interacting, communicating etc at the level we know he can do. Eg redirecting him from a repetitive/obsessional activity onto a more suitable one, and rewarding him for doing so.
If this sounds like it might work and you want more info I'm happy to give more info, as are many others on this board 