I'd look on eBay for 2nd hand small Islabikes or Frog bikes.
And if you can afford more, do - you'll get it back when you in turn put it on eBay
"New" bikes in that price range are incredibly heavy for their size (lumps of metal is my dad's description of them) and won't make it easy for your ds to progress from his balance bike and might even put him off.
Hello, My little boy is now pretty proficient with his 12 inch strider balance bike and we are looking for a follow on for his 4th birthday next month. Not intending to use stabilisers. On a budget of about £100, wonder if anyone can recommend something?