How can I check the history on my son's computer after he has deleted it.

I do not believe so.....you might contact your ISP and see if they can provide you with an account of what sites have been visited....

....I am assuming that is what you are after.....

If you wish to monitor sites visited in the future, you can enable Parental Controls. Depending on which version of OS X you're using:

10.5 (Leopard) and 10.6 (Snow Leopard)

http://support.apple.com/kb/HT2900

10.7 (Lion)

http://support.apple.com/kb/PH4522