The Mossberg twentytwos are fast becoming collector's items nowadays - that price was EXCELLENT! Any grungy .22 rifle starts at $150 in pawnshops down here. I'd have jumped on that Mossy! It would be a nice addition to my 8 other Mossbergs - I do not have a 151K yet.......
The 151K does seem to be more often seen lately than certain other models, but not in 'like new' condition.
But, for training your son, the Remington 514 is a good choice too - I love mine.
Check it carefully for proper extraction though - the extractors are often very worn on 514's. Numrich has them though, and replacement is a snap! I did that with mine.