(a) Yes, he still has a chance. He must immediately register himself for the product with Technical Manager (TM), Weapon & Equipment Directorate and User Directorate (and equivalent HQs in IAF & IN) as per the Format at Appendix B of Chapter II of DPP-2016.
(b) He should also respond to RFI as soon as possible giving valid reasons for the delay in responding to RFI.
(c) If procurement case is at an early stage of processing, there is every chance that vendor may be considered for issue of RFP. However, Vendor will have to put in extra efforts to convince user that he has a good solution that is worth looking at.