Hmmm, I'm not sure. It would depends on how well you could hear him if something happened/how big the house and garden are etc.
Dawn gave up naps before a year old and we had a poxy yard back then so I've never had to worry about it
I have nipped to the house next door when she was younger and in bed

, but I took the monitors (We spent about half hour the first time we did it, testing we could hear well on them

) But it was all terraced houses in our old house, so their livingroom was only really another few feet from mine anyway, iygwim!
Does his bedroom look over the garden? Could you leave the window open?