Racing is so dynamic that timing someone's average speed riding solo is not much of an indicator. Just enter him in a race and make sure he understands that he's likely going to get dropped his first time out. It has been quite a while for me, but I recall 10-12 criteriums being 25 minutes, over which time they generally covered 7 or 8 miles. As a 12 year old, I generally averaged 14 mph for a ~1 hour ride solo, so you can see the average speed in a competitive pack is noticeably faster.
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It's like riding a bicycle