Yorkshire Terrior is smelly after bath

I have a 3 year old Yorkie. He is a house dog and only goes outside for walks and to relieve himself. I take him for grooming once a month and bathe him myself after 2 weeks. I use Hartz flea and tick shampoo with oatmeal. After I bathe him, I use a blow-dryer on a cool-warm setting to dry is coat. He is smelly like outdoors in 3 or 4 days. This doesn’t happen every time but I was wondering if there is something else I could use to keep him smelling fresh until his next grooming.