What happens if the Contractor has not provided the performance bond while the project already on site and has completed 2/3 of the programmed works?
For whatever reason this matter seems to have been overlooked but I assume that the Contractor is able to provide a Performance Bond rather than not being able to procure one, for whatever reason.
If you have already discussed this then you could refer the matter to adjudication by serving a notice under clause 9.4.1. If the Adjudicator makes a decision in your favour, effectively for specific performance, then you could refer the matter to the courts to be enforced., if necessary.
In the case of Liberty Mercan Ltd v Cuddy Civil Engineering Limited  EWHC 3854 (TCC), Cuddy had used their best endeavours to procure a bond but it was considered as too much of a risk by the market. The courts ordered Cuddy to make a payment to the courts, who would effectively act as the Bondsman.
You could also discuss the possibility of the Contractor paying retention monies if performance is a concern. This would need to be a Deed of Variation to the contract, although would replace the obligation to provide a Performance Bond. If such a requirement is not needed at this stage then you could negotiate the payment of a sum of money instead (cost of Bond) which you would probably implement as a Deed of Variation as this would not constitute a Change under clause 5.1.