I shoot the 140TTSXs in both of my 7mags as well as my 280. (I'm kinda partial to 7mms ) The 7mm 140 TTSX is nearly perfect for all lower 48 game. I'd probably slide up to a 150 were I going for Yukon moose but I certainly wouldn't feel under gunned with the 140s, being a Barnes they'll still out penetrate 150-160 lead cored bullets!

As for the 120s, my son & his bud have piled up the does & yotes with them. I've yet to recover a bullet because everyone of them has been a pass-thru regardless of angle or distance. But I seriously doubt they'll stay in any whitetail based on my experience I had years ago with the .30cal 130 Xbullet I drove to near 4k fps from my Lazzeroni Warbird. I purposefully TRIED to blow it up or have a failure including double shouldering a 200#+ buck @ 25yds & another 200#+ I shot in the chest at 95yds as he was facing me while I was on the ground & drove him lengthwise. The Barnes just "keep going & going & going" sectional density doesn't really matter with a Barnes if impact velocities are kept over 2k fps. The lighter weights allow you to utilize higher muzzle velocities, flatter trajectories & in this case, a little less recoil for your sons 7-08 vs a heavier bullet.

Ps
I warn anyone about trying a 7-08... Be careful, you'll likely fall in love! I put together my sons 7-08 for myself actually but he claimed it for his 12th birthday before I could ever fire a shot through it. His bud likewise asked for a 7-08 so his dad had me put together a Ruger American for him. I had a 7-08 back in the early 90s but as a young, stupid man I traded it off. Now that I've been back around these "baby 7s" I've gotten "re-bitten" by it once more. So much so that my 7mags are likely relegated to safe queens & my Rem Custom Shop 280 will likely join them as I made the mistake of picking up a stainless Ruger American Compact 7-08. I'm waiting to pick up & feel a Kimber Adirondack 7-08. One of those two WILL be my next toy!!!