My son would not really take a pacifier at all until we introduced one called a WubbaNub. We just happened to bring it with us for our plane-ride home over Christmas, and he got addicted to it in like 2 hours! Its different than other pacifiers because it is a Soothie pacifier, but its attached to a little stuffed animal. Ours was a frog, but they have ponies, dogs, cats, as well. The stuffed animal has weighted feet that keep it from slipping off of his tummy if he is on his back, and also, it is GREAT for yournger babies who don't have too much controll of their hands because they can grab the stuffed animal part and hold onto it, which hold it into their mouth. My baby LOVED it that he could control it! He was the same age as yours (about 3.5 months) when we introdced it, and its still his favorite pacifier. They are a little pricy ($11-$15 each, plus shipping if they don't sell them in a store near you) but in my opinion it was highly worth it! The WubbaNub website is www.wubbanub.com and you can also search for them on google too. Hope this helps!
Oh and PS: There were nights when we gave him the wubbanub when we put him to bed, and the the morning when we woke up it was STILL in his mouth! We were amazed!