I would hit the fucking roof - selfish get.
£1,500 and 4 nights all-inclusive for his 40th, a Premier Inn and reused banners for yours - you don't need anyone to tell you the maths on that one, you've already done it perfectly clearly yourself.
You've spotted the pattern too, haven't you - every time your birthday plans came up, there was a reason to scale it back. Christmas is expensive, then flights are expensive, then wouldn't you rather have spending money than a nice hotel, then a cake's a waste, then the banners can just be reused. Each excuse on its own sounds almost reasonable. All of them together, compared point by point to what he had for his own birthday, tell a different story.
A few things worth sitting with:
This was never about being materialistic. You clearly aren't - you say yourself you don't give to receive, and you proved that by spending big on his 40th without a second thought. This isn't about wanting an expensive holiday, it's about the size of the gap between what you gave and what's coming back.
The gambling detail matters. Him having double your wages and still whittling your birthday down to a Premier Inn and no cake, while spending on online gaming, isn't really about affordability. It's about where he's choosing to put his money, and it isn't you.
The joke about reusing the banners is worth paying attention to. You said yourself you don't think he's joking - and jokes like that often are a way of testing how much they can get away with before anyone objects out loud.
Yes, you'd be disappointed - anyone would be. You planned an all-inclusive celebration for him without being asked twice, and your own 40th has been quietly negotiated down to almost nothing, at the same age, the same milestone year. That's not you being ungrateful or high-maintenance, that's a completely reasonable thing to feel.
It might be worth telling him plainly, before your birthday arrives, exactly what you noticed - the contrast, not just the details - rather than waiting to see whether the banners really do get reused. However he responds to that will probably tell you a lot.