Permanent tyre weld question.

Son has a problem where 2 of his tyres have slow punctures losing around 4psi per week. Plenty of tread on the tyres, is there anything like tyre weld which is permanent for slow punctures? Don't want to get charged £15 or so per tyre for a repair orveven just to take tyres off and try and fix if there is a cheap fix.
Thanks.