Do you mean that your son did his first practice paper (called Paper A) or his second one - called Paper 1. (Designed to confuse, of course!!)
Paper A is generally thought of as easier than Papers 1 & 2. However, nerves at doing real papers can really play a part in this, so don't panic.
On telling your son, only you can judge how he will react. Some children will be spurred on to new heights by a poor result this time, whilst others will fall into a pit of despair.
If he is at risk of the latter, consider being "economical with the truth" on how he scored. I have done that in the past - just added a couple of marks - and it has worked very well.