Have you looked into FODMAP it's a specific diet for IBS. Some high calorie options from that would be avocados, olives and good quality extra virgin olive oil, tinned coconut milk, coconut milk yogurt or smoothies - a good brand is coyo, coconut based snacks, 70% dark chocolate, higher fat meat, so rump and rib eye steak, chicken thighs over breast, all cuts of lamb, duck.
Tesco do a lactose free high fat Greek yogurt which is delicious and really high calorie.
Meal ideas;
Curry with a tomato and coconut milk base with either lamb or chicken thigh would be a good one throw in veggies you can handle, spinach maybe?? with brown rice (higher calorie)
Lamb meatballs and pea pasta (you should be okay with this) nice tomato and olive oil based sauce
Add seeds so pumpkin and sunflower seeds to lunches.
Do you think you could tolerate pine nuts?! Not strictly a nut, if so you can make your own pesto with lots of olive oil.
Cook / brown using either Ghee butter or coconut oil.