is he ok with milk? if he is ok with milk but only behaves lik that on solid food, mayb you should bring him to pediatrician, it could be sore throat. Just my guessing though.
and i dont agree with your mum that this is due to starting solid food at 5.5 mths. many babies started even later: at 6mth old but they are okay.
and sidetrack abit, i think your son is abit too early to try food lik cakes and even ice cream. =)