HI L.,
I don't think it is too early. I have two boys 17 months apart in age, now ages 4 & 6. At the time that I was pregnant with the second, I asked my pediatrician whether or not I should buy a second crib since my older child still occupied the only crib we had. She said to go ahead and make him move to a "big boy" bed and use the crib for the new baby.
So later when my baby boy was done with the bassinet, at 3 months, we bought a set of loft/bunk beds. On the same night we "promoted" the younger from the bassinet to the crib and the older from the crib to the bottom bunk (at age 20 months). It worked out great!
The type of bunk beds we got are called "loft" bed. They are not quite like traditional bunk beds, in that the beds are set up shaped like a "T" and sit perpendicular to eachother instead of stacked like traditional bunk beds. I think the traditional bunk beds will work as well, though.
I think success of this highly depends on your son, though. My 20 month old, at the time, did great. He never got out of bed once we put him down at night. I have a daughter that is turning three next month and she is still in that same crib. I KNOW she won't stay in a "big girl" bed, so we're keeping her in the crib as long as we can get away with it :-)
Good luck!!
L. Strobel