TOPEKA, Kan. (AP) — A Topeka man whose 4-year-old son was severely injured when explosives detonated in his car the day after July Fourth 2015 has been sentenced to 18 months of probation.

The Topeka Capital-Journal reports that Jacob Daniel Schell was sentenced Thursday in Shawnee County.

An affidavit says Schell’s son, Roman, was hurt after a sack containing “several improvised explosive devices” blew up in his lap. A Kansas Highway Patrol trooper says the ignition source was a “little cap gun” the boy was using. The explosion was so powerful that it peeled back the metal roof of the car Schell was driving.

Gary Conwell, Schell’s attorney, says his client thought the cap gun was empty and that all the “improper” fireworks had been used. Schell says it was a “terrible mistake.”