I don't altogether agree with the last answer. Whether he is still a guarantor depends on the nature and wording of the document he signed. If it was just that he was named in the first tenancy agreement as guarantor & signed the agreement as such, then his liability lasted only as long as that agreement was operative (because he has not signed anything since).
However, it is possible that the first tenancy agreement is still operative. At the end of the 6 months the tenant may have entered into a completely new agreement (in which case the guarantee ceases to be effective), OR the tenancy may have become a statutory periodic tenancy - i.e. it continued without any new agreement being signed. In this case the original agreement - and the guarantee - are still valid.
Another possibility is that he did not sign the tenancy agreement but a completely separate guarantee agreement. In this case, the provisions of that agreement will determine whether he is still liable.
All these points need to be checked to determine the position. If he is liable, the IP dealing with the IVA needs to be informed.