What punishment will a sex offender get for not registering when supposed to and is incarcerated at the moment with a bond? can they send him to prison for that?

Anything is possible. If a sex offender is incarcerated with a bond AND he has a violation for non-registration (or is he incarcerated BECAUSE he did not register), then it is more likely that the violation will force a return to prison to finish the time remaining from the early release - not to mention that if there are other charges on top of the violation.