My 14 yr old son broke his metacarpal 3.5 weeks ago. It has been casted from day 1 of break. What are the chances he can get his cast off tomorrow?

Ask the ortho. This question requires a lot of knowledge of the break, and a determination of how it's healing. I'm assuming you have an appointment tomorrow to assess this very thing. I would never plan on getting it off by scheduling something he needs it for the next day, but if it's healed well, there's a chance.
Good. Depends on the fracture and healing to date, but 3-4 weeks of casting is sufficient for most metacarpal fractures. After cast comes off often will use a splint for protection based on the activities he is doing.
It depend on the. Kind of break and were it is located, if the fracture in far end were joint are they could have the cast off to a brace, if it is in the middle of the bone I doubt that.
Depends. Your physician will determine if a cast or brace is needed based on xrays of the fracture and the results of clinical exam (ie: tenderness, swelling, motion). The fracture may not be entirely healed, but using a brace may be a possibility.