No offence. While the road is officially closed through a proper closure order it ceases to be a highway for duration of that order. Were he in a motor vehicle he'd then commit the offence of driving other than on a highway, but he's not in a motor vehicle so no specific offence applies.
Were the road open, then the usual motorway rules would apply, and he would be committing an offence.
Now I'm not sure about motorways because I don't deal with them (because we can't cycle on them) but is "a proper closure order" for them a Works Traffic Regulation Order or a Temporary TRO? Violating those normally carries a fine of up to £1000, according to the boilerplate on our locals.
Of course, many are unenforceable against cycling because they routinely fail to sign the closure at entry points from cycleways, but that's hopefully irrelevant for a motorway which shouldn't have any entry points from a cycleway not already covered by motorway signs (such as a cycleway crossing a motorway entrance). We're lucky that our cycle route signs and motorway signs are different colours... oh wait, no, that's France, Belgium, the Netherlands...