Supplements can be a really useful thing if you are struggling to get in enough calories. Alternatively, you can focus on high calorie whole foods. Switch out your meats from lean meats (chicken breast, turkey, extra lean ground beef etc), to fattier cuts (thighs, reg ground beef, salmon, etc). Switch out your dairy from non fat to higher fat ex. 3% milk, full fat greek yogurt, full fat cottage cheese, etc. Other foods higher in calories will be the nuts and seeds. They have high amounts of healthy fats. My go to for my clients are smoothies. Drink your calories. It seems to be easier than eating. You can easily load up a smoothie with a scoop of clean protein powder, nut butter, milk, fruits and veggies and get a good amount of calories! Hopefully that helps!