I know this has been covered recently, but couldn't find anything on those threads that quite met DS1's needs.
DS1 (6.5) can ride a bike very slowly with stabilisers (though the very slow bit might be to do with the fact that it's a crappy bike ). He absolutely refuses to try a scooter or a bike - we've offered to buy him a new bike (with stabilisers) but he just says he doesn't want one, won't ride it etc etc. He has dyspraxic traits so his balance and steering aren't great, but what has really clinched it for him was seeing an adult come off their bike quite nastily when we were driving about a year ago.
SO - my latest strategy is to buy a three wheeled scooter for DS2 (5) in the hope that DS1 will just try it one day. We were in a toy shop on Saturday and found one with two wheels on the front. DS1 was quite keen to scoot it round the shop, with us pretending madly all the time that he was 'just trying it for DS2'. We would have bought it, but it was pink and covered in fairies!
Does anyone know of something similar that would be suitable for a 5-7 year old? DS1 is about 122 cms I think - might be a bit taller.