Yeah, christmas has time. Roots won't be strangled for about a solid weeks and some change of strong fast growth.When he transplants, he'll know if he waited too long by the shape of the root ball. Does it completely encircle the container? Too long. But the 3.5 gallon pots could hold it until flowering if it's a small plant. If it gets too big for it's britches, transplant to a 7 gallon, or even a 10. With a 10 gallon pot you could grow a full sativa 12+ feet tall, no biggie. That's with hydroponic growing, though. I think loam is too much of a limiting factor for such large plants. Could be wrong though...