Upvote:4
He might get refused entry even with a visa if it appears he is coming to the UK to get NHS treatment. He would need to demonstrate that he has very substantial funds to cover the possible need for private care.
Dialysis costs something like £300 per session. 3 sessions per week is standard but new patients may need fewer sessions initially.
The fact of being an amputee is no issue.
Disregarding visas and costs, he may find the trip very strenuous (some of my elderly relatives really hate long haul). You should consider having your sister visit him as an alternative.