Hello and thank you for your post. 

I can understand you want advice, given his current situation.

Unfortunately, as nurses, we cannot give you very specific diet advice. You would need to speak to the hospital medical team or else ask to obtain a referral to speak with a registered NHS dietician. They might suggest particular supplement drinks/food products for example. 

There are some resources that I can signpost you to though,

Macmillan Cancer Support have useful Building Up Diet information.

Sometimes it is possible to able to get a prescription for different drinks through a GP. We have some information on the different types of drinks that you can get here

The Royal Marsden Hospital have produced a cookbook on nutritious food for patients with cancer which you might want to purchase.

Do make contact with his hospital treatment team, or clinical nurse specialist (CNS), and ask for a speech and language therapy (SALT) referral as well.

I hope that this helps,

Best wishes,

Vanda