Exaggerate the sound slightly when you say it. Because bilabial speech sounds are among the most visible sounds, it is easy for a child to see you put your own lips together to produce them. For example, if the child is saying "ilk" for "milk", you can respond with, "You want mmmmmilk?" Kneel down, if necessary, so you are face to face.
Moo like a cow, baa like a sheep. Children love animals, and the names of animals are among the first words in a child's developing vocabulary. Using toy animals or pictures in a book, talk about how a cow moos, a cat meows, a sheep baas, a chicken buck-bucks. The child may not be able to name the animals, as sounds like "k" (as in cow and cat) and "sh" (as in sheep) develop later, but he may be able to make the animal sounds. The American Speech and Hearing Association (ASHA) recommends using the child's natural play environment to expose children to sounds. For example, ASHA suggests the following: "Blow bubbles and make the sound 'b-b-b-b.' Feel the motor in your throat on this sound. Engines on toys can make a wonderful 'rrr-rrr-rrr' sound." You could also make an "m-m-m" sound for a motor.
Put a dab of honey on the child's lips. Using a sticky substance, such as honey, peanut butter or syrup increases sensory awareness of the lips. You may need to use a mirror and model this for the child. Smack your lips together with the honey. Smear it between the lips. Practice saying "mmmmm." Note: Always get parents' permission before using a food substance, and ask about peanut allergies if using peanut butter.
Practice lip popping to encourage production of "p" and "b." The word "pop" itself encourages the lips to come together and then quickly open. Using the back of your hand or the child's hand, reveal how there is a puff of air when "p" and "b" sounds are produced.
Talk to your public school's speech and language pathologist. Many public schools offer services to the preschool population, and some offer services for infants to 3-year-olds. A speech and language pathologist can determine if something physical is interfering with the child's production of speech sounds.