My sons are same,(21&16 now) they were 900g&950g when born 3 months early,youngest one has had a lifetime of drs,dieticians,etc,who incidentally said dont worry about WHAT he will,just try feed him up,when he gains weight he will grow.she was right.and like your grandson he dislikes certain foods,due to textures,wont have veg at all,only fruit apples,but only very occasionally,doesnt like sauce or gravies of any kind,and will only ever drink cola( though he will have sugar free,but not diet version)has milk occasionally,but not regularly,(theres loads more i could add to his list of dont do's or dont wants) but when he was 15 they found he has mild cerebral palsy and aspergers is listed on his medical notes,though no one ever mentioned that before,my elder son,also has no sauces,gravies,etc,has to have his foods separated,but will now at least eat some veg,but not others,likes plain foods,meat, fish,but only chicken without bones,not in any gravy or sauce just dry,wont have butter or spread unless on toast,but will at least have a variety of drinks, he had mild cerebral palsy from birth but has OCD and ASD issues in some things,mostly to do with textures,hes about average size & weight now,and youngest is now growing taller than older one since he left school.Both are capable of most things,got GCSE's,are at college and university now,older one driving too.Children are amazingly adaptable and im told they adapt their diet to suit their own preferences of what they can& cant eat,( my youngest also seemed to 'shape' his diet to what did/ didnt cause tummy pain) after 7yrs the drs diagnosed his reflux problem! Try not to force different foods on your GS, he will only get upset,then he wont want to visit next time.im sure his mum and dietician will be on top of this,and even the one new taste he did try is a big breakthrough,its enough for him for now.