Firstly, I hope you're doing well, a CABG is a major operation, so congratulations for getting through! I should point out I am a trainee doctor, and not yet the finished article(!), but I hope I can give you an accurate insight.
I think your consultant was right to raise his eyebrow, February is very optimistic, and in my personal opinion, not a wise target. Depending upon where the vein was harvested, you could have a reasonable amount of oedema (swelling) in your leg and ankle, which can take up to 6-8 weeks to resolve completely. The sternum takes around 6 weeks to heal properly, which is most people's major stumbling block to efficient recovery. You simply can't do anything involving heavy exertion during this time. I'd then expect you to need a couple of weeks (at the very least) building yourself back up to being able to reasonable exertion.
To put it simply, the fittest person imaginable wouldn't be able to get about fully for 6 weeks, and even then it would be pushing it, which puts you into mid-February. But the fittest person imaginable wouldn't have had the procedure in question? Could you give me an idea as to your age and otherwise general health (BMI etc)?
I'd be saying back to work in 6 weeks-ish if your job isn't too strenuous, but refereeing certainly is! My personal advice would be to concentrate on relaxing and recovering, and then once the rest period is over, follow your planned rehabilitation (if that involves stress testing, etc) and gradually build up your fitness levels. If you are refereeing on the opening day of next season, you would be doing very well. Anything earlier, and either you're taking a risk, or you're Superman!