Completely understand that you're not comfortable with either! I didn't sleep while DS slept on me - but I knew if he slept then the next day would be easier than if he didn't. I wasn't going to sleep while he was crying all night, anyway. I didn't know he wouldn't sleep in the cot since he had previously without issue. We co-slept only because we were away for 4 days and I just couldn't handle not sleeping for that long. I can't nap through the day, so it was that or nothing.
My DS is a terrible sleeper, so if he refuses to nap the first time I put him down (which isn't often these days), I get him up after 40 min and try again later on. You could try that if you really want to teach him to nap in the cot and aren't too concerned with whether he has a sleep at the right time.
@wobbleyhorse has a good point. It could work somewhere else. Does he sleep in the pram when you are out at all?