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