Why is my dog vomiting mucus and foam?

My 5 month old chihuahua first began vomiting his food about two days ago. He hasn't wanted to eat anything since then, he has no energy and doesn't play anymore. Last night he started vomiting mucus and his mouth gets foamy. I read that it can be because of the weather change because its been raining but hes been sick for 3 days now. He has his first parvo shot, could he have parvo or what can it be?