That's great if one can wait until their child is "ready" for changes like moving to a bed or potty training, but it's not always realistic. If you don't want to buy another crib (and who would?), then start transitioning your son now. Most everyone I know says you don't want to wait until after the baby is born, because then, the older child associates getting kicked out of his bed with the baby.
But, it you have the room, don't waste the money on a toddler bed, just get a regular twin or full bed (whichever you eventually want him to sleep in). If you get a toddler bed, you'll have to buy another crib size mattress and the toddler frame, then in a year or two, you'll have to buy a whole other mattress and frame when he outgrows the toddler bed. Also, if you, one night, have to lie down with your son because he's had a nightmare or is sick, it's really miserable for you to lie down with him in a toddler bed.
I know because we put my 2 1/2 year old son in a toddler bed in preparation for the arrival of his baby sister last month. I'd rather have gotten a regular bed for him, but we have space issues (so looks like we'll be getting bunk beds when the kids are older too!) My son has a lot of energy, takes a long time to settle down, and still does not sleep through the night, so the transition was difficult. So, I started him in early December (26 mos. old) and he finally slept in it around mid January. Until then, he'd have a hard time, would sleep on the floor or fall asleep in my arms or in my bed, with me moving him after he fell asleep. So, yes, I did have a challenging time with the transition, but I was also potty training him from November until late January and sickly pregnant too. So, lots of transitional changes all at once. But, now he's wondering why his baby sister doesn't sleep in the crib in his room, since only babies sleep in cribs...
Good luck to you and congratulations!