Welcome to your baby giving you his opinion! We too have a baby boy who is almost 6 mos old. These last few weeks have been rough - almost every nap be puts up a fight. I can't say that he HAD been sleeping great, and now it's changed, but I can say that as soon as he THINKS he's being put to sleep, he starts arching his back, squirming and screaming.
Because of this, we've started to let him Cry It Out a bit. We have been "teaching" him that he can't always be held, rocked to sleep or carried around.
Sometimes when it's time for his nap, I put him in his bouncy chair. He goes through his normal PO'd spell, and I just keep going about my business saying, "You're ok" or "I'm still here", letting him see my face, maybe smiling at him, but NOT picking him up or "rescuing" him. I think he's starting to get it. He's learning to self soothe. Once he starts to settle down, I will pick him up and move him into his crib. He may fuss a bit, but usually he falls asleep within 2 minutes now.
I don't know if you have him on a schedule...for naps and bedtime, but that really helps our LO to not get so overtired. He seems to be better about "expecting" a nap, and knowing that I'm not going to come in and save him. So he settles down faster each time. And if he wakes too early from a nap, I don't run in to get him. I listen to his cry and sometimes leave him so he can fall back asleep again. If they get overtired, then they seem cranky all day and really fight sleep HARD. If you don't have a naptime or bedtime routine down, its good to have a few cues to help him understand what's coming. Even if it's as simple as closing the blinds/drapes, turning on a sound machine and singing one song.
I'm not big into the CIO methods where you just walk away and leave them entirely. I have found that BEFORE bedtime or naptime staying close to him so he can see me while he winds down works well. But in the middle of the night we've been letting him either CIO or my DH helps him. I breastfeed, so as soon as he smells me, he wants to eat. We have been working to go down to 1 night feeding. So if he wakes other times, we've been letting him fuss a bit. If it goes on too long or escalates my DH will go in to help him.
I hope this helps you. Feel free to msg me if you like.