"He has seven sons."
Translation:उसके सात बेटे हैं।
पास is not used to indicate possession when you are talking about being related to someone, having a friendship etc. It is mainly used for inanimate objects, animals or people you have employed.
When you do use पास for people, it can mean something stronger than just 'have'. For instance there is an old iconic Hindi movie quote, "मेरे पास माँ है". For a little context, two brothers in the movie are arguing and one of them lists all his material possessions to say how rich he is. The other brother retorts with this line. The line is not just saying that he has a mother but that she is close to him and on his side.
Should it not be "उसको सात बेटे हैं"?
As @vinay92 rightfully pointed out, it's always के when followed by पास or साथ
But we are using neither पास nor साथ, so it should be incorrect to use उसके without पास.
Although "उसके पास सात बेटे हैं" gives a slightly different meaning, it is still grammatically correct.