Sadly, it's going to take time to get your son used to sleeping alone in his crib after all this time sleeping with you. Getting a baby to sleep on his own is difficult no matter what age you try to do it, but since your son is already older, you have that much more habit to break. When we were trying to get our son to sleep in his own bed, we did a couple of things that made it easier. However, since he was only 3 months old, you'll probably have to work longer and harder to get everything to work. You should probably wait for a weekend or some time when you can make up the sleep, because originally, you're going to lose some.
First, establish a bedtime routine, and stick to it WITHOUT FAIL. For us, we do dinner, then a bath, then a story, then some rocking with soothing music in the background. This prepares your baby for sleepiness, and it really works! Once he has figured out that he does the same pattern every night, he will start to get sleepy when the pattern begins. Our son starts to yawn the moment he hits the bath. Second, go ahead and rock your son to sleep - later, you'll want to rock him until until he is very drowsy and then put him down, but to begin with, that is probably too much to expect. When he is asleep, set him in his crib, and leave. When he wakes up, allow him to cry or fuss for 5-10 minutes (no more) before going to him. If he wakes but does not fuss, let him lie in his crib until he starts to cry, then head up. Pat him, stroke his hair, or rub his back (or whatever helps him relax), but don't lift him out of the crib, make eye contact, or speak to him. Give him 5-10 minutes of crying before you pick him up again and rock him again. Repeat as many times as necessary. You can change his diaper or whatever you need to do, but it is important to avoid eye contact and talking to him once you have put him down to sleep. He'll stall you to get your attention, but if he learns that during nap time, he doesn't get it, he'll be less interested in staying awake.
As for the weaning, you're right. Most babies can sleep through the night by the time they're 6 months old, long before they are weaned off of breast milk, so chances are your son isn't actually hungry at all at night. He is either not eating enough before bed, or he is using you out of habit and to pacify. Make sure he gets a big dinner, and then decide that you are going to feed him once during the night (probably around 2-3 is when he will get hungry if he is genuinely hungry). The rest of the time, don't offer your breast. Give him a pacifier or go ahead and rock him back to sleep. You really can't give in and take him back in your arms to sleep, though, because then he's just learned that next time you try to break this habit, he just has to fuss longer and harder.
It's a difficult process, but it is worth it. And although a couple of nights will probably feel very long, the quality of sleep that you will get will eventually make up for it!