AIBU to 'change my mind'.....?

[whatsthepointinwasps]

Ok so this is my first ever post but I've followed some of the other AIBUs so here goes....
My youngest is 18yrs old so not technically a child (oh yeah right hahaha)
I'm off work with a nasty chest infection and feeling pretty
grotty but not totally incapacitated (can still move about, just slowly with lots of rests)
I was lying on my bed with Nflixs when my son (said 18yr old) came in and asked me what I wanted for tea, I was touched and surprised he offered so I asked for tinned soup and banana sandwich.

He asked me if I wanted it now so I said 'yes that'd be nice thanks' and he went downstairs.
He put on his music and there were noises from the kitchen.
Fast forward nearly two hours .....no food appearing....
I ventured downstairs to find him in dining room on his laptop with the remnants of his meal beside him. 'Er I thought you were going to do my food too,' I said......'Oh yeah, I'll do it now,' he replied.
Another half an hour passed.....he appeared at door of my room....
'Can you just do your own tea as I don't feel like doing it now,' he said.
'But you said you would, you offered to do it in the first place' I answered.
He said 'I know but I've changed my mind now.'
The thing is I wasn't expecting him to offer to cook for me but the fact that he did then was so blasé about his 'change of mind' actually hurt me.
I feel this is dragging on so I'll cut to the chase (thank god finally I hear you all sigh)
Would it be unreasonable of me to wait till the next episode of 'Mum would you...' or Mum can you....', initially agree, make him wait a while then say 'Well actually I've changed my mind now'?
Is that petty? I had been planning to rustle up the soup/sandwich myself before he asked anyway.

Should I just forget it? Put it down to 'He's a teenager and it's better to pick your battles'?
Sorry about the length of this but if anybody who has struggled through to the bitter end would care to comment I'd be most grateful

Vince thanks so much for your more personalised reply
Firstly I definitely under reacted at the time-that's because I was struggling to breathe and speak in sentences at the time so didn't have the energy for any confrontation.

As for letting him away with selfish behaviours, well usually we 'discuss' it me rant he shout back on the spot; then when we are both calmer we talk it through properly. He seems mostly to take on what I say even if his way of showing it can fall short of a full on apology.

Yesterday he offered voluntarily to help with some tidying jobs, taking extra care with them, supported me round the supermarket, put away the shopping while I went back to bed then made my tea. He went out his way to get me my fav crisps too (unasked for) 'to make up for last night' and I was touched more by that than by a meaningless 'sorry'.
This might be little recompense in other families but it was genuine and 'off his own bat' and to me that's what matters most.

That outcome is lovely OP - I too had thought you under reacted and wondered if you effectively enabled him to treat you in that way but clearly not. It sounds like he is genuinely sorry and has gone about making amends in the right way so all is well. In fact it is probably a better outcome than the shouting approach would have achieved! I hope you are fully recovered soon.

Vince is right, my responses where to Duchess not you and I apologise if it came across that I was criticising. It was clear that you were surprised by his behaviour which suggests it was a one off, and his apology confirms that.

Great final outcome, although ultimately not helpful to you in your hour of need!