You likely will never know due to privacy issues. There are a lot of possibilities: criminal behavior, institutionalized, successful adoption, reunification with birth family...to name four. The last two are way more likely given age.
Based on what I'm reading, that doesn't make much sense. According to the source article:
"DCF documents show that he was a court-appointed special advocate volunteer for CASA. CASA partners adult volunteers with foster children to provide emotional support as they go through the court system.
Those documents show Capps was assigned a 5-year-old boy. Last year DCF removed that child along with other children from their foster home. Shortly after, the foster mother filed a complaint against Capps saying he caused emotional abuse for two boys."
It seems something was wrong with the foster home.
"He was also accused of babysitting the woman’s children, which CASA said is in violation of his duties."
That makes the least bit of sense. If children are removed from her care, why is she in contact with the CASA of one of them?
The initial investigation said something was wrong. The appeal said it was unsubstantiated and talks about paperwork.
This whole story has a whole bunch of oddity to it.