Thank you for your tips, fantastic!
Depending on what your dog's skin can handle, bathe him every two to six weeks. You don't want to bathe them every day, or even weekly, because their skin will dry out as well as their coat.
If you want your dog's hair to grow out, it is very important to brush him each day. You can find spray-in conditioners for dogs at a pet store or website that sells grooming supplies. Also buy a conditioner for bathing the dog, as that will help eliminate tangles and mats.
As for the dog getting muddy, that is expected of such breeds with fine, thin hair. They are meant to be indoor dogs for the most part. To help keep the coat clean between baths, you can also get bath wipes; they're like baby wipes. That way, you do not have to bath the dog, and after wiping down the fur you can comb it out to avoid tangling and matting.