N.N.
You defnintely have tried it all and I am feeling for you. I'm just going to start from the beginning and hopefully something will help. How is his night sleep compared to his naps? Does he night sleep in the same place as his daytime naps? If all his sleep is in the same place and his night sleep is fine, then he may know that you will go to him during the day.
Health- is he sick? congestion? ear infection? gas? etc.
Naptime routine- keep a consistant routine to signal to him that it is time for a nap. Change diaper, close blinds, sing lullaby, etc.
Timing- avoid getting him overtired. Try starting the naptime routine 15-30 minutes earlier. Look for sleep cues.
Growth spurt- at 6 months, he may be having a growth spurt. Extra feedings for a few days may help.
Environment- too cold/hot? turn on/off fan? darken room? age apropriate transitional toys in crib? My son loves the Halo sleepsack. It's a zippered blanket that stays on him and he nibbles on it to soothe him to sleep. Maybe the airvent is blowing directly in the crib and disturbing him?
Does he like a pacifier?
When he cries, do you go in and soothe him back to sleep or go in and end the nap? If the latter, he may want your company instead of sleeping. CIO works but it is dependent on how consistant you are with the method and which method your child responds to. Healthy Sleep Habits, Happy Child is a good book to read. My son did not respond well if I went in to soothe him no matter 5-30 mins into CIO. So, I put him down and watched the viedo monitor until he finally soothed himself to sleep.
Sorry to be redundent. I hope something helps your son gets some sleep.