My dad died of carcinomatosis which had started in his prostate. My mum died of cancer within a month of her dx.
It is terrifying when a much loved parent gets a dx of cancer and what you want to do is give them absolutely anythingnthat you possibly can to make their lives better, more fun, more interesting whatever. If mum had expressed an interest in a Kindle she'd have got one, a solid gold Rolls would have caused problems but luckily she wasn't that type.
I definitely wouldn't have balked at the idea od spending £100 or so on something, I would have thought, well, how many holidays/whatever do we have ahead of us, we can manage on a lesser one or even none at all for the moment.
I had little control over the utter desperation I felt to make her last days better and it took over all my emotions and thoughts.
I do realise that the situation is not quite the same, but we didn't know how long she had to live, either. We just knew that her days were definitely numbered. I have no idea how long this state of desperate would have continued because mum died v soon; probably it would have calmed down after a time, but I'm also pretty sure that things would have brought it back to the surface - birthdays and Xmas definitely, but little things too, like she might be looking more tired one day, or be weaker or something.
I would talk to dh first (mostly) but I think I would have been very bullish about it and he would have had to agree whether he did or not, iyswim.
OP, your dh should have spoken to you first, but I suspect he would have got it anyway. There is some over-riding imperative forcing your actions which is very very hard to resist, and mostly you are unaware of it and pretty helpless when in it's grip.
It is possible that seeing his dad on his birthday brought it back into play (how many more birthdays will my dad have? He doesn't look/seem well, he looks tired or old etc etc etc all act as prompts for the remembrance that he has had cancer and will not be here forever and he is my dad).
I do understand how peed off you must feel about the expense and the subterfuge, though. I hope that my post might have some relevance to your dh's feelings and go some way to showing how he might have been feeling and what prompted his actions.