if his current H1B is from company and H1 time under 6-year-limit, he does not need a new H1 number because this is extension. Also, a Singapore passport holder can get H1 number reserved for Singapore citizens.
Any way, if he wants US GC, he should join your petition as a derivative, and if he is not China-born (Singapore citizenship is not enough, GC looks at birthplace to charge the immigrant visa number) your I-140 category visa number can charge to his birth country to use the worldwide cut-off date. If he does not join your petition category, you will still be subject to the China-born visa number waiting list.
