Kids are pretty good at getting out of things they don't want to do sometimes. If your son doesn't like school or riding the bus, it looks like he's done a good job of trying to avoid them. However, since his tactics are at least successful in making him late for school, he's not going to have any desire to change. It's important to find ways to motivate him to be on time for the bus.
Problem-Solve the Issue
Sit down with your son and engage him in problem-solving. Talk to him about what he doesn't like about riding the bus and going to school. Make sure he isn't skipping out on the bus because he's being teased by older kids or something of that nature. I've worked with plenty of kids who dread riding the bus because they don’t have any friends to sit with or because other kids are misbehaving. If he suggests there is a problem of this sort, work with him on finding solutions to resolve it.
Also, address the problem of how slow he is to get ready in the morning. Talk about whether or not he may need an earlier bedtime so he is better rested. Or discuss strategies that can help ensure he is ready on time, such as packing his backpack and having his clothes ready the night before.
Establish Negative Consequences
Make sure your child receives a negative consequence for missing the bus. Finding what will work to deter your child from missing the school busy depends largely on your child's temperament. Perhaps removing a privilege, such as all electronics (TV, video games, computer, etc.) on the days he misses the bus would be effective. Find a privilege or activity that will bother him if it’s taken away and don't allow him to have it on the days he misses the bus.
Sometimes it makes sense to establish some logical consequences. For example, figure out how much money you are spending on gas to get him to school. Then, make him pay you for the cost of the gas that you are using to drive him to school. If he earns an allowance for doing chores, deduct this amount from his weekly allowance. It can be a great opportunity to teach kids about money as well.
If he doesn't earn an allowance, you can still have him pay you for the gas money. Set up chores for him to complete to pay off his debt. Don't allow him to play video games or do fun activities until the chores are complete and his debt is paid off.
There may also be an opportunity to allow for some natural consequences. Depending on how far the school is from your home, making your child walk to school may be an option. I've worked with parents who have followed their child to school in a vehicle for safety reasons while their child walked.
You can also try rewarding your son for getting ready on time and for making it onto the bus. A simple reward system may motivate him to try and get ready faster in the morning. You could also try to use a token economy system where he earns tokens for getting ready by a certain time and for getting on the bus.