I don't think there will be any impediment to giving him an extended tenure without a break. Before making a decision, consider these aspects as well:
1. What is the ultimate purpose? If you decide to extend his service for further periods, you may extend with a 'break' or without a 'break'. 'Without a break' means it would be a 'continuous service' and enable the employee to receive gratuity until his retirement, including the extended period. This unbroken extension may be opted for by the employee, which is favorable to him; it may also impact EPS.
2. You may need to revisit your terms & conditions/HR Policy to make proper amendments.
3. If you decide to effect a break-in-service, it will be considered a new innings, and consequently, his terms & conditions have to be redrawn.
4. If the above point is the decision, you have to arrange for F&F settlement.
5. This dispensation may be cited as a precedent in the future.