Advice please

my 14yo child has just received an ASD diagnosis. The last few years have been extremely hard, with my child suffering severe anxiety. 

They are now receiving help and have been prescribed sertraline to help the anxiety, which is starting to work. But they are still suffering from liquid diarrhoea every morning and during the day. Their sertraline dose is still low (only 50mg) and we are going to up it to see if that helps, but any other advice on how to help their gut would be great. 

We have looked for links with food intolerances etc., but they have this diarrhoea every single day, it doesn’t come and go. It also only started as their anxiety got bad. But that does mean they have been living with an incredibly upset tummy for over a year now.