I have a 21-month old and am expecting in three weeks, so I understand completely about not wanting to buy a new crib! My daughter has been sleeping in her "big girl bed" (toddler bed) for a month now. This is what we did. We put the toddler bed in her room so she could get used to just seeing it. We told her it was a "big girl bed", but really didn't make a big deal over it. After about a month we asked her if she wanted to sleep in the big girl bed and we let her sleep in it just for her naps at first, which was the recommendation of my ped. She slept in it for a week for her naps and then we just let her sleep in it at night. It was an easy transition. The crib is still in her room, but we are taking it down today and will put it up in about a week or so in our room for the new baby. Hopefully, when the new baby starts sleeping in it, she won't want it back!
If the timing doesn't quite work with transitioning your son out of his crib before the new baby arrives, don't panic and buy another expensive crib. We had the co-sleeper bassinet by arms reach as a back-up plan. We used to use it when we traveled or went to grandma's. It is less than $100.00 and they can use it until they are about 3 or 4 months old. Good luck and congratulations!