How is it going now ? Hope he’s a good eater by now !

I like the comments so far.

I was always told to not stress about it too much as a child will never starve themselves. Keep offering food at regular meal times, he’ll eat when he’s hungry enough. The more fuss you make, the more he’ll keep doing it

A 2,5 year old is very much in the stage of discovering it’s own will and personality. You say he has been extremely fussy with his food and only eats certain food when he feels like it and also swishes his food or water in his mouth until it turns into liquid and then spits it out.
First of all I would give him that piece of independence where possible.
I would make a tray with little healthy snacks for him (ice-cube trays are handy) where he can eat from throughout the day when he feels up for it. You could fill the tray with pieces of cheese, boiled egg, pieces of meat, fruits, vegetable muffins/bites, fritters, yoghurt, etc.
Personally I wouldn’t slap a child because it doesn’t eat in the way and time I want it to eat. Instead of going into battle I would give the child a bit more space. Most children wouldn’t starve themselves indeed. Have his weight checked when you have concerns and/or ask medical advice when you see he’s is starving himself indeed.

first of all if he is swishing food with water take away the water so he can’t do that.
as the other mum who answered said don’t stress he wont starve himself. however if he has a mental or developmental problem then he actually might (i know of an autistic child who did), if this is the case the seek professional help.
what we did for my nephew who was doing the same at about 1.5 yrs was sit him in the high chair and watch him as we ate so he saw us eat and when he tried to take the food out of his mouth we said loudly and clearly in a grouchy voice “no” “food stays in your mouth” if he spat it out anyway he got a smack on the hand and again we said ‘NO’ “naughty” he quickly stopped pulling food from his mouth and eventually just swallowed it because he had no other choices.
took about two weeks of doing this every night and the problem was solved.

Don’t over stress it, he won’t starve himself. It sounds very frustrating though. Just keep persevering