Changing plans with short notice

[RainbowAura]

DP of 3 months asked yesterday morning to see me that night. Nothing special, just going round to his house. He messaged me less than an hour before I was due to set off to say he wanted to do something with his housemate instead and did I mind? By the time I got to speak to him it was past the time I would have been setting off. I said I did mind and thought the short notice was shit (I'd planned my day around it, got ready etc.) I wasnt going to force him to see me if he didn't want to, so didn't go over.
For background, we've been getting on great. He makes me feel really cared for. Things are moving nicely and we got back from holiday together a couple of weeks ago.
He also previously said his pet hate is letting people down and hates people who cancel last minute. He's 33.
So, AIBU to feel let down?

YABU - this wouldn't bother me actually
YANBU - this would piss me off a bit

No, that's just plain rude. I wouldn't be impressed.

As a one off I would be disappointed but I wouldn’t mind. Although it would have been better if he had actually phoned and spoken to you rather than just messaging. If it was a regular thing I’d probably dump him.

YANBU. This would piss me off monumentally, not just a bit. I'd make a mental note and cut my losses if it becomes habitual.

Lucky it was you and not me.
I hardly ever look at my phone.
I would have turned up there!
It's bloody rude.
I would not be impressed.

That would piss me off more than a bit

I've just sacked someone off for exactly this, except it had been going on for 12 months. It started off with him cancelling things last minute, but nicely. I didn't kick up a fuss so he then realised he could walk all over me. It got to the point where he'd just stand me up all the time and then get annoyed with me, if I dared to question him about it.

Considering he asked you to go and see him in the first place, I would be mightly pissed off at him changing his mind again.

Thst's pretty rude.
Did he expand on what it was he wanted to do? If it was a once in a lifetime opportunity type thing or his friend had a problem or bad news etc. That would be fair enough.

'* wants to do something with me tonight. Is it OK if I chill with him?'
Which I took as code for go out and get smashed as they both like a drink.

That is rude

Arh, yeah that's not on.

It would piss me off greatly and let me know that he's really not interested/invested in the relationship.

Three months in its shit, it's honeymoon period and he can see his housemate anytime. I'd dump him, in these times nights with other half's have a short shelf life.