If you had bought him his first ever computer, a few years back, would he feel as if he had to keep using it forever, or would he too be wanting to upgrade, ideally with your support?

I think you're being incredibly patient and considerate with his feelings, probably far more so than I would manage under the same circumstances. Good luck, and I hope you can come to an amicable arrangement when the time comes for your new bike. If nothing else, it's a great example to set to your children ("dad bought mum bike, mum loves and rides it so much that she's now able to have an even better one and the old one still gets ridden too"). Enjoy your cycling!