OP, I'm so sorry to hear this. I have been in chemo for AML this year but I can confirm that it very much depends on the type of chemo he is having.
The first (induction) rounds of chemo for my kind of leukaemia are done as inpatient treatment - they were horrible and I really did feel terrible. But even with that (and my team told me that it really is the worst type!) the ill effects didn't kick in properly for a day or so. It sounds like your DP's treatment is happening as outpatient, so it may well not be so bad.
My practical advice would be to ask for anti sickness before the sickness actually kicks in (they often give it as a matter of course anyway) and, if the side effects start up, ask for extra straight away. It's kind of like car sickness - if you can head it off at the pass, it can just go away or at least recede. If you try to tough it out, it just gets worse!
Some people find that distraction helps. I couldn't read or watch anything - it made me sick - but listening to podcasts or audiobooks can distract and be soothing.
Boiled sweets or ice cubes can help also, but I'm sure that the team will advise.
For a lot of people, the worst is the fatigue and that doesn't really kick in for a few days. So you should be okay for Christmas!
Good luck with it all
I would agree with you not to put it off. If you do, chances are you'll both be worrying and won't enjoy it anyway. Focus on what you can do - you'll probably both feel worn out and emotional just from the logistics and stress, so be kind to yourselves and plan to do things which are relaxing and which you can change if needed.