Check with the airline. Depending on the severity of the condition he may need prior approval. He also may need an escort, for example if he cannot make his way to and with the toilet.
In the US it is the airlines responsibility to provide assistance but in the EU++, it is the airport. However, in both cases, request the assistance from the airline and, where possible, 72 hours in advance.
Avoid connecting flights if possible. If connecting avoid U.K. airports - a search here will tell you why.