回複：wellcome notice?

It is normal. It generally happens to derivative benificiary. If your son needs a stamp on his passport, you can make an appointment (via infopass) and bring the "welcome to USA" notice to Local USCIS office to get his passport stamped.

By the way, I had to go to local office to stamp my passport when my I-485 was approved. My wife received the welcome notices from TSC and received her GC in one week.

Good luck and happy new year!