My lo is 5 months also and started fighting naps last week after being really easy to put down for months. He was crying and fussing and wouldn't settle or calm down. I don't know if this will work for your lo but I found that if I massaged his feet with one hand during our winddown time and cuddled him close (usually during his story) that he settled down and when he was really calm I put him in the cot and he went to sleep easily. If he started fussing, I got him out and settled him again. It's working every time for us and trust me he was really fighting going down. See if that helps.
Oh, another thing that has worked in the past with a really overstimulated baby is to take baby to your bed, darken the room, and curl up with baby really close to you, pat on the back and try to get to sleep that way. The only problem is you can get trapped with sleeping baby and wakeful parent!
