Hi - Suggestion regarding picky eater, especially regarding the amount of milk. Try diluting it. Start gradual - maybe only an ounce of water - and every couple of days, add a little more water. He'll get used to it. What percent milk do you use? Try going to a lower fat level as an alternative to the diluting. If he drinks whole milk, go to 2 or even 1%. He does need some of the fat in it but not necessarily whole milk.
As to the food issue, maybe he doesn't like the texture of the food in the jars. Give it to him fresh or partially cooked. Besides, the jar food is more expensive as it's a convenience food, so going to fresh is going to save you some money. Nothing wrong with his taste buds. They all go through it. Mine got hung up on hot dogs and mac 'n' cheese for what seemed like eternity!!
You need to put your foot down on grandma and grandpa giving your baby soda!! It is SOOOOOOO bad for them! Pure sugar. Even worse if they give him diet soda. Most diet sodas contain Asparatame, which has two pages worth of side effects! They include weight gain, depression, hyperactivity, mood swings, and on and on and on. The soda, though will rot his teeth before he has any. He's your child and if they won't abide by your rules, then they should not be watching him for you. Yep - give them the ultimatum.
Do you read to him? Babies LOVE to be read to and you will be amazed at what he picks up on! We used to even go through mail order catalogs, calendars, etc., and identify the items to my son. He's 13 now and is above average in his learning skills. If you have the means, get a library card and make it a weekly event for him. He'll grow up appreciating it.