A little more information about the patient could get you more answers!
1. Diagnosis? Brain damage? Dementia?
2. Capable of conversation? Understanding of the situation? Medication?
3. Aim of Physiotherapy? Is it just to get him out of bed in a chair and leave him there? Could the aim be changed?
4. Preparatory exercises with participation in bed prior to sitting if possible on the edge of the bed. Can he turn over? Can he sit on the edge or does he hyperextend there already ? Does he "Push"? What are active movements like in bed? Hypertonus? Hypotonus? Stiffness? Pain?(we are back to diagnosis and assessment)
5. Think of neuro basics whether Bobath or whatever. Start on a appropriate participation level that is possible without panic. Without a proper assessment, we can't treat effectively. Advice online is rather difficult anyway.
6.And on a slightly weird note: Lying "very comfortable in bed" was he? Thinking of all the geriatric patients I forced out of bed to walk when a young physio, I decided in a weird way that I, as a geriatric someday would probably say "no way" if forced out of bed when I didn't want to and didn't see the point in it. Of course it's good to be mobilised and up. But one can mobilize and do physio in short suitable stages with intermediate goals as well. Even if the NHS policy dictates otherwise!
Wish you luck
Ancient Physio. Still getting up by herself.