Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned about Leo's 5 hour car ride.
Cats are prone to car sickness as their sense of balance is much better than ours and as such they are easily affected by the motion of cars or boats. Some cats do outgrow it, but some don't. For future car rides it may help to try to acclimate him to the car. Try to keep trips short and as positive as possible. Some cats get stressed if the only places they go are negative, such as a veterinary clinic. By making the destination enjoyable he may feel less stressed so less likely to get sick.
You can give Gravol also known as Dramamine (dimenhydranate) for carsickness. The dose is 4mg to 8mg per pound of body weight. Give it an hour two before travel. It will last about 8 hours. Side effects are mild sleepiness and dry mouth.
Or you can try Benadryl (diphenhydramine) at a dose of 1mg per pound of body weight orally (1/2 of a 25mg tablet per 8-15 pound cat). Side effects are sedation and dry mouth as well. It should be given an hour or two before travel
I do recommend that he not eat for 8 hours before travel since the car-ride will be long. Ice cubes in an attached bowl in his carrier will melt slowly and if he licks them can help to quell nausea too and keep him hydrated.
I also recommend keeping a window cracked as fresh air seems to help too.
Some cats do better if they cannot see out of the car, the moving landscape seems to make them dizzier and thus more nauseous. Try putting his carrier on the floor of the car.
To help with anxiety you can try Feliway (which is a synthetic version of a calming pheromone). It comes in a spray as well as impregnated collars. Another option is a calming homeopathic remedy called Bach's Rescue Remedy. It can be used along with Feliway and Benadryl. See this link for further information: http://www.bachflower.com/Pets.htm
Please let me know if you have any further questions.