There is no reason to be streased he can enter on tourist and do his voluntary work, no one is arresting legit volunteers from legit charitys he can do it on any visa for time being untill his tourist visa runs out and needs to switch. Be real here, in 13 years I have no ever heard of a volunteer or legit charity being harassed by immigration the ones being abused by visa mills are easily distinguished everything else is jobsworth advice.