Dad not eating

Really struggling at the minute, my dad had an ilyostomy in September due to a blockage and was diagnosed with non Hodgkin lymphoma in October. He has had 2 lots of chemo at 50% and one at 100% but is now not eating or drinking. We have been given the protein drinks but he has gone off those and is only drinking about a pint of liquid a day. He doesn't always take his medication either. I'm at my wits end now as to what to offer him as I can see he is losing so much weight so I'm worried he won't be strong enough to tackle the next lot of chemo (the cancer has shrunk with the chemo he has had which is positive). Any help or advise will be greatly received.