Eating and drinking issues

Hello, My name is June I am a grandmother of a 3 yrs old Boy who is Autistic.

8 weeks ago he started refusing drinks, to the extent of not taking any fluids at all. We over come this by giving him watery foods , such as yogurts, fruit pots , puddings and Weetabix with lots of milk.

He became ill 2 weeks ago with a virus and fever, so stopped eating due to the fever , his GP sent him to hospital but nothing was done they just made sure he was not dehydrated and sent him home .

He has recovered from the virus , BUT  the situation has heightened over the past few days as he is not eating his usual foods now.

We realise this appears to be a sensory issue but we do not know what to do and how to get support. the GP does not seem to know what to do. 

Is there anyone out there who can give any advice or guidance , Please xx