Any tips or tricks?!

So my baby is really small and not gaining enough weight each week. We see a GI specialist and he said if we come back in 6 weeks and he’s still dropping off the chart then he will need a feeding tube. He is ebf and refuses a bottle. I started him on solids at 5 and a half months per doctors order and he does okay with the food that he likes. He has has sweet potato, carrots, green beans, peas, butternut squash, apples, bananas, strawberries, mango and asparagus. Any tips or tricks to get him to eat more or take a bottle? I have breast milk stored so I have tried that. Thank you!