There is no one size fits all re activities etc. Toys that spin or turn may engage him e.g. spinning tops, puzzles with cogs, roll a ball toys ( going down a spiral ramp) etc
Bead track toys are often popular.
If he’s attending Nursery now the staff may observe any difficulties with speech and language skills, social interaction skills and sensory needs. They may speak to parents and suggest Speech and Language therapy and outside agency involvement such as a specialist teacher.
Re food, please be led by parents and your Grandson. When you say he gags on food you know he likes, the difficulty may be related to texture, brand or even how it’s presented. Please don’t introduce new foods or hide foods as this can lead to refusal of accepted foods in children with restricted diets.
You could research ARFID or Sensory Processing Disorder. Both are diagnosable but you may find tips for activities and dos and don’ts.
You could do ready steady go activities and anticipation rhymes pausing for a reaction. Repetitive rhymes and books are good. He may enjoy interactive board books especially tactile ones or Things That Go books. Some may have wheels that turn.
Sounds like you will have lots of fun together.
