Or something like that.
Kizmet, you mentioned no problems with 2x4s on 24 inch centers. But what you described sounds like a truss, not an ordinary ceiling joist.
I think he should be ok with the 2x6 with 16inch centers as long as he doesn't have a long span.
My personal experience was with an in-laws garage ceiling with a 20 foot span with 2x6s on 16 inch centers. My Father in law had approximately 1000 pounds of material spread evenly over the attic, and oner a 30 year period, the joists sagged approximately 2 inches.