Baby rejecting fm & bm.

Hi mummies, since LO was 3 months old. There will be days where he refused to drink fm & bm. He will cry and push out milk bottle when we try feeding him. Doctor says LO might had been sick of milk already and suggest sleep feeding. We just went for baby 4 month old vaccine recently and doctor gave us green light to feed him solid. Since yesterday the same cycle repeats and LO was refusing fm & bm so I am thinking of giving him solid. Any suggestion on what kind of ingredient is most suitable? Will baby be able to swallow mash apple or banana? How many times per day can I give baby solid?