I agree with Nai - if there hasnt been an improvement cant hurt to head back to the GP.
I know how awful this can be, I remember DS1 first case of gastro (at about 9 months) and it went for a week, it was awful how he wouldnt feed etc and threw everythoing I did get into him back up. I went to the dr a few times.
I am sure you know this but look for wet/damp nappies - at rhe very minumum a damp nappy every 6 hrs. the hyrdalite will help with electrolyets as well as fluid so keep that up. there is also 'pedialyte' which is for kids and i know that was slightly easier to get into my DS1 (also with a syringe). i weas doing 5-15ml at a time, every 20 mins or so.
when he is able to eat/drink again - breastmilk will be best (you could express and try to get him to have some in the syringe also) - in any case express for your supply??? in terms of food - apples are good for the pectin (helps ease runny poos) and also bananas. plain rice... avoid dairy/meat for a while except maybe some yoghurt w probiotics. sweet potato is also a good food to sdtart off weith - mashed if he will do iut or i used ot bake in 'chip cuts' which he would eat as finger foods.
He will put the weight back on, i wouldnt worry too much - it is amazing how the appetite will return when he is well. but i know how worrying and i really thinka trip to the gp wont do any harm and will either give you peace of mind or will ensur eyou dont end up w dehydration etc.
good luck, let us know how it all goes.