So, he'll be about 12 weeks when you get him? I would just make sure he's completely weaned before you bring him home. Tiels typically don't wean until 8-12 weeks but some can take longer. It's worth waiting the extra time for him to be fully weaned. Also, baby tiels sometimes regress even then over the stress of a new home and need some hand feedings. So you want to weigh him and make sure he's eating properly when you bring him home. Also, if the breeder clips wings- which I hope he doesn't- I would encourage you to ask the breeder not to do so. Baby tiels need to fledge and learn to fly properly.
Best diet for tiels opinions vary, but I shoot for about 50% seeds, 15-20% pellets and the rest vegetables. Baby tiels are also typically clumsy. So for the first couple weeks you might want to keep the perches low in his cage in case he falls. Tiels should get 10-12 hours of sleep. They are also prone to night frights. So I always have a night light on at night for my tiels. Congrats on your new friend.