Gaining weight after chemo

My dad is struggling to gain weight after his chemo & pancreatic surgery (he had the Whipple procedure). He is deemed cancer free, is eating all the time but cannot seem to gain weight. He is eating lots of protein, drinking an ensure, adding calories wherever he can and still cannot gain. He is somewhat limited in what he can eat as he also has Crohn's disease & IBS. Any advice on what could be going on?  He is 7months post surgery & 5 months post chemo. Any help/advice is appreciated.