Dear Elizabeth, all babies develop at different rates. Some babies start walking at around 9 months, while others may not start until they are almost 14 months; and yet, all that difference is within the normal range.
For babies developing within the normal range, there is no need to do special exercises to encourage physical development. One of the best ways for you to help your son develop the muscle and coordination he will need to accomplish appropriate developmental skills, is to provide him with opportunities to practice his newly acquired skills. First, make sure the environment in which he plays is baby-proof and safe for him to explore. Being there to encourage and support your son's emerging physical skills, while allowing him to experience the trial-and-error of doing it by himself, are important in developing his self-confidence and problem-solving skills. When he begins to show signs he is getting ready to crawl, you might encourage that physical skill by providing toys that move away just a little when touched, so your son will be motivated to go after them. Or, when your son begins to pull himself up to a standing position and shows signs he is ready to try to walk, you could provide toys that support him and encourage him to take those first steps.
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.