Sounds like your little guy is giving you a compliment. He wants to stay awake and spend as much time as possible with you on the weekends! First, I would ask the daycare center what they do before a nap time... Even if he does not nap at a set time, they must do something. After all, he doesn't just collapse on the floor when he's sleepy!
The big thing I think is to "feed the meter." Switch off with your husband every half hour or so, but make sure the baby is getting somebody's undivided attention all morning. He'll be more likely to let you put him down for a nap if you've "fed the meter." (I think I picked that advice up from Happiest Toddler on the Block by Dr. Harvey Karp?)
If he's started crawling, etc... he may need more time to let his muscles wind down before a nap, so think about non-movement activities to do before nap time. You might try eading books and carrying your son around the yard for maybe a half hour of quieter activity before a nap.
I also found that my son would go down easier after a "new" kind of experience. For example, if I let him do something unusual, like going through a normally locked cabinet. Or if we spent the morning playing in several different rooms in the house, upstairs and downstairs, with distinctly different toys. Good luck!