You do not understand insurance correctly in this context. It will not cover any defective workmanship. It will likely insurer him against injuries caused on your property by his work/workers. His insurance protects him and not you.
As has been mentioned above, many home insurance policies include legal cover and also have a legal helpline, check your policy. They can initiate a legal claim on your behalf if there is a good prospect of success. The offset is that you have a claim registered against your policy.
The best thing you can do is find a template for the first stage of small claims court and follow the process - send a recorded delivery letter stating the problem (defective roof he built), your expectation (roof repaired), a timescale to rectify (2 weeks), and the consequences of non compliance (you will get another to repair and seek to recover all costs from him via court).
As has been mentioned above, many home insurance policies include legal cover and also have a legal helpline, check your policy. They can initiate a legal claim on your behalf if there is a good prospect of success. The offset is that you have a claim registered against your policy.
The best thing you can do is find a template for the first stage of small claims court and follow the process - send a recorded delivery letter stating the problem (defective roof he built), your expectation (roof repaired), a timescale to rectify (2 weeks), and the consequences of non compliance (you will get another to repair and seek to recover all costs from him via court).