Is this top good for really cold weather??? I want somehting to keep him warm

It's a great base layer, but not great in terms of warmth. Use this with an insulating layer over it (I prefer down), finished with an outer shell. EDIT: Sorry, misread the date of this post wrong, doubt this is much help over a year late :P

This really is only a baselayer in cold weather. For perspective, I have 150 weight shirts like this that I wear as a t-shirt in the summer time. For winter I often wear this shirt with a 320 weight wool sweater over top, and if it is below freezing, I'll still likely need another insulating layer underneath my shell.

It only is if you are going to wear another merino layer over the top of it (and then your insulation/fleece and shell). If you aren't looking to have too many layers I'd go for something heavier as your base in the 200, or better yet 260 range, and then go straight to the insulation layer OR if you really want to keep nice and warm add a expedition weight 320 to the mix along with the 200/260. The good thing about merino wool is it regulates heat very well so you don't have to really worry about overheating. I've hiked with a 200 and a 320 on in temps that went from 45 to 80 and sunny (with elevation going up 1000ft) and still been fine with the sleeves pushed up. EDIT: I'll add that the 2nd layer over this one should have a nice zip up neck, keeping your neck covered will help keep you a lot warmer AND that zipper can be lowered to dump heat if you get a bit too warm.