emma, I appreciate it is hard to just not feed him if you worry about him losing weight, but it really is the quickest way to get him to eat properly in the long run. By adding stuff to his food you are going to make him more fussy and he'll end up eating less in the long run. Plus, sweetened youghurt isn't good for him.

You really should take him to the vets if he has had the runs for a week.