So if the constitution has nothing about taking away criminal's rights, do criminals in prison have the right to free assembly? Losing rights is part and parcel of committing crime. Don't want to lose them? Don't commit crime.
This guy had every right to defend his son. No question. But his past choices ruled out some means that the rest of us have available to them. It's pretty hard to abduct a child when a baseball bat is rearranging your cervical vertebrae.
Having said all that, one of the biggest problems with our legal system today is how many silly, petty crimes are felonies. That needs to be fixed. But not by unrealistic policies like pretending serving time erases history or restructures one's judgment and predilections for the better.
And his gun had the serial numbers filed off. Back to jail, buckaroo.