I know it’s not longhaul, but years ago 5 weeks would have been perfect for the Greek Islands by using the extensive ferry network. I would also consider Thailand and Vietnam but everyone goes there, don’t they? I guess because it’s set up for backpackers. Also, again years ago, the Greyhound buses connecting USA cities was a favourite. However the cost of living there is now a bit expensive.
Megabus cheapest European travel or book cheap flights, the earlier you book the cheaper. Sleep on bus or train or plane at night, it saves cost of accommodation. You are responsible for your own safety at home or abroad. Must have travel insurance at point of booking travel eg before departure. Travel light, take tent
Indonesia is very cheap - hostels/homestays £7 per day & local food £3 for good meal and local restaurant. He could start in Java, travel to Bali then ferry to Lombok. Lots to see/do in Indonesia and flights can be found for £400 return.
Asia for cheap accommodation but flights are expensive. He could go interrailing which is really flexible and he won't need to plan his itinerary in detail.
I travelled in Canada for a summer. I worked for each hostel for a couple of hours a day handing out leaflets and housekeeping, in exchange for the night's stay so it's possible to keep accommodation costs down that way.