dh i'd echo what mlic (albeit not as bluntly
) - you need to keep consistent for naps and night-time sleep if you want to see an improvement.
F was very much the same as D and I battled for months to get him to have a daytime nap in his crib. Eventually (at maybe 5/6 months?) he did but gosh was it painful progress til then! I used to spend an hour settling him for him to sleep for 20 mins! Once it had finally clicked, he always took a really nice long nap so I was very pleased I'd persevered.
Also, if you don't let him cry it out then it's likely you'll do your fair share of unplanned co-sleeping (esp for illness and teething). I wish someone had told me that in the first year of having F as I always felt like it was a kind of guilty secret and I was a failure because I couldn't get him to sleep thru in his own bed!