Another one tandem feeding a toddler and baby (2.5 DD and now 20 week DS). I echo what others have said about struggling with feeling touched out. I was really surprised at the negative feelings towards my toddler that feeding created as previously I had only positive thoughts, and I just wasn't prepared for it. But it did pass and I now really enjoy my breastfeeding snuggles with my eldest - it has definitely helped with the inevitable sibling rivalry, and it's also helped remind me that she's still a baby too in lots of ways. I think it's easy to see the toddler as this big grown up when baby arrives (they certainly suddenly look huge), but they really are still so small and for me having that closeness together was really special and an important reminder that she still needs me just as much as she did before.
Whilst it's 100% your decision, as you are due at Christmas, I do think it's not the right time to wean - he will remember that he used to get your milk and it might intensify his feelings of jealousy as he then has to watch the new baby at your breast instead. I think if you wanted to wean, the time to have done it (or at least to have done it while minimising any consequences) is probably past as it'll be in his active memory. Not helpful I know, but I can't help but think you're potentially exacerbating his natural feelings of jealousy by removing the breast so soon before the birth.
On the practicalities, I actually found it easier to assign the kids a breast each - so the toddler could feel that she still had her special part of me, even if she often couldn't have a feed as immediately as she'd like. I found that helped with her patience as the baby wasn't at 'her' breast, drinking 'her' milk, so she didn't find waiting as difficult. She calls it 'her boob' and knows it's there, with her milk, even if she has to wait to get it. However, during her nap and overnight, the baby had free access to both (shhh... don't tell her!), which has avoided any issues with supply on that breast. In fact, it helped manage the fast letdown that the toddler was creating on both breasts at first, as once I assigned them a breast each it meant the baby could manage its own supply and demand on his own breast - it would come out so fast it would nearly drown her, poor thing!
In short, I'm so glad I am still feeding my toddler alongside my baby and without doubt think it's helped her cope with the transition to having a sibling. It's also helped maintain our closeness when the truth is my attention and time is now so divided and she doesn't come first anymore - our breastfeeding snuggles give us those special moments just the two of us. I have no intention of forcing her to wean and will feed her as long as she wants.