Please help! sick parrot!?

I came home from camp (after 4 days) and my sister was supposed to take care of my sun conure. when i left him he was really healthy and all. when i came back, he was hanging by his foot from a toy and he looked really weak. his water bowl was empty. my sister said in the morning he was fine. but she had forgot to refill his water bowl out of all the instructions i gave her. so he didnt have waer for 4 days. i helped him off the perch and hes drinking so much now but he looks SO WEAK!! doesnt seem to be injured but cant fly and can barely walk. and he squeeks instead of his usual squaks! he doesnt have energy to eat all he is doing is drinking and im wrapped a towl around him and he keeps snuggling with me.... PLEASE HELP! i think he got dehydrated or something will giving him water heal him again? im gonna try to take him to the vet but my mom said only if its life threatening no matter how much i reason with her. she says just give him water. i need to know if hes gonna be okay if i just giv ehim rest and water - pleeeease no rude comments im on the verge of tears