I need help making my father gain weight

Hello everyone at r/weightgain!

Now I know the title is a bit odd, but stick with me. My father has recently been diagnosed with MND (or ALS). He cannot eat a lot in one go and tends to "graze", having smaller meals every hour. However, he still isn't eating enough to maintain weight and has lost 2kg in 2 weeks and apparently patients with MND tend to fair better if they are slightly overweight.

Now I know we should see a dietician and that is getting organised soon, but for now, I was wondering if any of you fin folk would be able to give me some tips on fattening meals for him? The lunch also needs to be able to be eaten without being heated and on the go because he works on a tractor for a good portion of most days.

TLDR: Dad has MND, has lost 2kg in 2 weeks and needs to be kept slightly overweight for best outcomes, cannot eat big meals but only smaller meals more often, what are your suggestions?

Thankyou guys for any help. I would really appreciate it.