Well first off what does caffeine do for the body, like chuck stated it has many great effects. First off what most people are looking for is burning calories. It helps mobilize free fatty acids in your blood so that they are utilized as a fuel source faster, meaning they don't get attached to your body as fat. It is also an appetite suppressant, curbing those cravings you get from time to time. It helps retard exhaustion and helps with recovery after lifting/exercise. meaning you can workout harder, and longer and not get as tired and sore from the workout. It also changes your perception of how hard the exercise is, making it seam like it is easier to lift the weight. It allows endurance athletes to perform longer and harder and has been shown to increase strength in weight lifters.
The down side of caffeine; its a diuretic, meaning you need to drink extra water to stay hydrated. It increases ones blood pressure, so if you have high blood pressure normally and take caffeine before your workout, you may have a higher chance of a stroke or heart attack when you lift (lifting also increases blood pressure). It can make you jittery and uneasy, some people just don't like how jittery they feel while lifting and all day long. It can cause an upset stomach for some people, making it hard to get a good lift in if you have to run to the bathroom all the time
Research can show caffeine helping or inhibiting atheltic performance, you have to know how to dissect the article to see if its a legit peer reviewed article or one written by the supplement companies, so be aware of what you read.
Now on to the nitty gritty, how much caffeine to take. nutritionist will recommend as a dietary/atheltic supplement take 2-5 mg/kg body weight (1 kg = 2.2 lbs) So if you are 220 lbs, or 100 kg, you could have 200 to 500 mg of caffeine a day. A standard cup of coffee or red bull drink has about 80-90 mg of caffeine in it.
As for your TDEE (total daily energy expenditure) or calories burnt throughout your day, you need to know your RMR (resting metabolic rate) lifestyle and activities and the thermogenic effect of food (calories used to digest the food you eat). your RMR can be done by going to a university or some gyms that can test it, the rest you can get off the internet by plugging in your weight etc into a formula to spit out a number. Once you get all three of these numbers you can figure out how many calories you truely need. by using the formula chuck gave you there can be quite a bit of error. if he was off by 500 kcal/ day you would be putting on one lb each week and vice versa, if he undershot it by 500 kcal/day you would be losing 1 lb each week.
As for the drinking of soda before lifting, that may be for the sugar, to spike the glucose levels; many endurance athletes will drink flat soda before and event to get lots of quick energy from the sugars... or it may just be that he is stupid and drinks soda before lifting
My professional opinion is much like patty in that you need to cut out some of your cardio in order to keep your mass, or just do intense interval cardio for around 30 to 40 min max a couple days a week. and of course if you want the mass you need to have your reps down below 8 with quite a few sets and heavier weight to start the hypertrophy phase of lifting.