My eldest Tonk boy was like this when he was younger. I found feeding him on Whiskas made him worse - I switched him to Applaws and RC sensitivity control and that sorted him out.
I have to admit I have found the Tonks to have very delicate tummies although my girl - who is now 5 mths - has been fine but she has been on Applaws and Orijen since I got her.

I would try the bland foods with him and get the vet to check him over, you should be able to get the RC SC31 from the vets to try and see if it helps any.

Welcome to the forum xx