Thank you all so much for your lovely replies and helpful thoughts.
Noodle - DH has been getting a bit impatient about me continuing to feed DS in the last few months, I'd say. He wants to know when I plan to stop. I don't even know the answer myself, but I know both I and DS are very happy continuing at the moment. I don't want to rush him, because it's so obvious that he gets a lot of comfort and reassurance from it. I get annoyed by people IRL hinting that I should stop now because DS is too old for it. I go to LLL meetings regularly, so I know I'm far from an extremist in feeding a mere 13MO!
DH told me tonight he feels jealous of the bond I have with DS through BF. I can understand that, because it is such a special thing, but it's not a reason for me to stop.
I hadn't thought of suggesting my mum lets DS watch tv. I'm a meanie and almost never let him see any, and he stares at it mesmerised when it's on, so that might actually help her in a pinch!
I think I'm concluding that my hotel idea is a rubbish one. DS would surely be much better off at home all cosy in his own bed, instead of traipsing up to London with us in the freezing cold. The sling idea might help too. He fell asleep in ours today when I was wearing it, and my mum has been known to use it.
Truth is, I don't know what would happen if I wasn't there to solve the problem for a few hours. Eventually, DS probably would go back to sleep, but I just hate him getting so upset if I'm not there. He really loses it. It's happened a couple of times in the past when we've tried to go out for an evening. Some practice runs are a good idea too. I thought we might experiment with DH putting DS to bed from time to time, to see if they can figure something out between them.