I would contact a vet and see if they suggest inducing vomiting. It may be they will tell you to observe him for any signs of poisoning.
Tomatoes, potatoes and rhubarb contain oxalates, which trigger abnormalities with the nervous system, kidneys and digestive tract. Especially toxic are green tomatoes, green potatoes, potato skins, and tomato leaves and plants, can trigger tremors, seizure and heart arrhythimia, so tomatoes should never be given to dogs.
Read more at Suite101: Toxic Foods for Dogs: Learn What Foods are Poisonous to a Dog
Toxic Foods for Dogs: Learn What Foods are Poisonous to a Dog