I'm so sorry to hear that Jackson is still struggling and with solids as well. It may be that Jackson has some bad connotations with food or he could be a sensitive child. Our nephew has never had any food allergies/ sensitivities or reflux and he is a very fussy eater and has quite a limited diet.
My only advice is to keep it as low key as possible, stick to the foods he does like and try new ones but have the liked food on hand if he rejects it.
We used to mix apple into new foods because that's what Dom liked and it made the new food familiar.
Some people use distractions like toys and the telly and find that it helps with a fussy eater.
Just perserve it will probably improve when his reflux goes.