Erin, some 5-month-olds find playing on their tummies natural and enjoyable, and others don't.
In the past 10 years or so, since health experts have recommended putting babies to sleep on their backs to prevent SIDS (Sudden Infant Death Syndrome), we've found that babies tend to spend far more time on their backs and less time on their tummies. As a result, babies get less experience on their tummies and are slower to learn to lift up their heads and chest, which can be frustrating.
But doctors believe it's important for your baby's development to have some tummy-time during the day—not necessarily most of the day—to increase your baby's head, neck, upper body, and arm strength and enable him to learn to crawl and explore his environment.
Your baby will enjoy tummy-time more if he can see you and other interesting things. Try placing your baby on his tummy on the bed, facing you, with his head off the edge of the bed—you can play peek-a-boo and he can see you easily without having to push up his head and chest. Or lay your baby on his tummy on the floor, with a small cushion or rolled up towel propping him up under his chest—you can lie down in front of him to play with him, and set out interesting toys within his reach. (Of course, you must never leave baby unattended when trying these exercises.)
Try to encourage him to push up with his arms by showing him interesting things above his head (such as your face, a mirror or rattle). As his arms and upper body become stronger, he can begin to enjoy playing on his tummy on an activity mat. As his lower body also becomes stronger, he will learn to crawl forward to reach interesting toys that you set out for him.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.