He could have been hit by a flying metal pipe during the explosion having walked in just before the explosion hence no gas in lungs. I suspect an explosion muddies the forensic waters just a bit.
unless I'm misremembering, there wasn't any explosion. They broke the gas pipe and left, making it seem like they died of suffocation (except for pipe guy, obv.). And when Ian and Becky went there, there was no sign of any explosion or fire, the shopfront was perfectly intact, remember?