Unfortunately, I think that this scenario falls through the cracks of what is allowed by any kind of temporary leave to enter the UK. Here are the possible routes, and an explanation as to why your contractor won't qualify.
Business – general activities
5 A visitor may:
(a) attend meetings, conferences, seminars, interviews;
(b) give a one-off or short series of talks and speeches provided these are not organised as commercial events and will not make a profit for the organiser;
(c) negotiate and sign deals and contracts;
(d) attend trade fairs, for promotional work only, provided the visitor is not directly selling;
(e) carry out site visits and inspections;
(f) gather information for their employment overseas;
(g) be briefed on the requirements of a UK based customer, provided any work for the customer is done outside of the UK.
Since the visitor would be giving these lectures as part of a course to which you're charging students for, he can't qualify as a business visitor. You said that "technically" this is a free bolt on to the course, but I think that is a legal fiction that is irrelevant here. The point is that without having paid, your students wouldn't be going to the talks.
24 An employee of an overseas based training company may deliver a short series of training to employees of a UK based company, where the trainer is employed by an overseas business contracted to deliver global training to the international corporate group to which the UK based company belongs.
Since your students aren't employees of a single UK based company which is contracting the training, this doesn't apply.
Permitted paid engagement
(b) An expert may give lectures in their subject area, if they have been invited by a UK Higher Education Institution; or a UK based research or arts organisation provided this does not amount to filling a teaching position for the host organisation.
If you aren't a qualifying organisation, then this isn't a permitted paid engagement.
(c) An overseas designated pilot examiner may assess UK based pilots to ensure they meet the national aviation regulatory requirements of other countries, if they have been invited by an approved training organisation based in the UK that is regulated by the UK Civil Aviation Authority for that purpose.
You've said that your visitor is an aviation expert, so this paragraph could be applicable. Obviously, you'll know if you're an approved training organisation.
None of the temporary work visas are applicable, unless the visitor is in the arts or entertainment industry, or is a sportsman. It's certainly possible I've overlooked something here, so I'd encourage you to consult with a solicitor.