Did your doctor express concerns? If not, I wouldn’t be too worried, though it might make you feel better if you get a second opinion.
And for what it’s worth, it’s almost impossible to force a baby to eat if they really don’t want to. And even if you could, it’s not wise unless they’re failing to thrive. You don’t want to set the scene for a battle. It’ll make mealtimes hell if you do.
As far as foods that could pack on poundage…you could try some full-fat dairy now probably - not milk, but full-fat cottage cheese, yogurt, mozarella cheese, etc. My daughter has loved string cheese since she began eating solids - it’s a really handy, healthy snack to have around. You could also try meats (ground chicken, ground beef, etc.) if you’re not vegetarian. Those options aren’t necessarily guaranteed to pack on the weight, but they could help. If you are vegetarian, maybe some beans?