My DD now almost 17wks was a complete car seat addict. She would refuse her crib during the day (was fine at night), initially she would only sleep on us and then one day I popped her in the car seat and she learnt to get herself to sleep and was even able to self settle, that's how it began. When she got to about 10weeks (I think) I realised that this could be a bit of an issue when she went to nursery, abeit not for a long time yet. So I started to google how to get your baby out of car seats. I came across some information about how it is bad for their spines (MIL has spinal problems so it made me think) and also how it can effect their breathing because they chins are quite close to their chest. How true this is I don't know but for me it was just more fuel to the fire to get her out of it, whether it was true or not. DD would nap for almost 3 hours (twice a day) and so I just didn't want to risk it.
So I decided to go for the same tactic as I had used to get her into a bedtime routine. At 6 weeks we started bath, massage, milk and bed and it took 7 days to complete and by the end she would just drift off to sleep at night most nights. The first time we tried it it took us about 5 hours to get her to sleep and it was exhausting, it gradually got better. So at nap times I would take her up to the crib and pop her in, I tried it for a week and had no luck, the most she would nap for was 30 mins and she was exhausted. I was getting a 30 min nap out of her each day and no amount of singing, rocking, patting etc would soothe her. After that week I was exhausted, run down and got very unwell, lets face it, you spend a whole week trying to get a baby to sleep it's going to knacker you out!! DH came home put her in the car seat and she slept for 4 hours! It was sheer relief, so we went back to the car seat and for 3 or 4 days she slept for long periods while she caught up on her sleep but then all hell broke lose. She literally stopped napping, she wouldn't nap at all in the car seat or anywhere else, it was like she'd forgotten how to nap. It was horrendous because I'd take her for long walks and nothing, I'd stand rocking her for and hour and nothing. One day she screamed with me trying to calm her how for an hour.
I felt like I'd destroyed all of her good habits by taking her out of the car seat. DH was off work for a week and so we decided to tackle it together but it got worse. I think that could have been due to our inconsistant approach though (he did one thing I'd do another). So when he went back I thought right I've got to tackle this. I did some more research and someone had made a comment that if you're trying to get your baby to nap and not having luck then do what you do at bedtime. Now this will obviously be harder for you if you LO just won't let you put him down. Now obviously I wasn't about to bath her at every nap time but I realised that at bedtime she is fed before she goes to sleep. So I tried BFeeding her with no luck, she still screamed the second I put her in. Then I realised that at bedtime DH gives her a bottle of (6oz)formula and she was obviously panicking that there was no bottle. So I made up 1oz of formula and gave that to her and followed it by a BF. Once she was dozy I would pop her into her crib and she just knodded off!! I was quite stunned. She knew exactly what she wanted, it just took me a while to work it out.
I must admit that I'm now stuck in a different rut...feeding to sleep. It seems to me that with some babies each time you find a solution you step into a new problem! So I'm gradually going to witdraw the formula and just BF and then hopefully BF a while before nap time. haha like that's going to work but I keep trying.
My advice to you would be to start a bedtime routine. You don't say how old your DS is? We started ours at 6 weeks and it was painful at first but now she LOVES bedtime. If you can get that to work then hopefully naps should follow if you follow the same process for naps. It's entirely up to you of course, I know bedtime routines aren't for everyone and lets face it what I'm having to do isn't ideal so I hardly advocate it for other people to do. It's all swings and roundabouts. The best piece of advice I am learning is that you do what you have to do to get through it. I stopped worrying about what I thought I should do and suddenly everything got so much easier.
Good luck.