The query does not disclosed the actual reason for suffering so it is difficult to know whether it is justified or not and if justified then what is the best remedy.
Stepping up of salary at par with junior is very simple in Govt. It is a routine matter and matter of right. If any odd department rejects this right one can move to CAT.
If the applicant is not getting the relief from deptt then either his case is not so simple or artificially complicated by deptt (or even himself).
Your query is not very clear. it does not reveal :-
(i) Whether Mr X is having lesser salary than Mr Y.
(ii) Whether both hold same designation
(iii) Is Mr X having a prior Sl No as compared to Mr Y in any seniority list.
(iv) Whether the seniority list is still operative.
(v) Whether the higher salary of Mr Y is not caused due to (a) his earlier officiation /joining on the post (b) his already higher salary in the lower post before promotion.
(vi) Mr X has not suffered stoppage at EB or any penalty affecting the salary.
(vii) Mr Y does not have any personal pay due (i) higher qualification (ii) Family planning operation etc.
If the answer to these questions is in favour of Mr X then please elaborate on which arguments (which you have not speficied) department is defending the case. Whether they are actually defending or just getting lazy.
In case the arguments (if any) given by the department in the reply (if any) to the representation (if any) are not legally tenable as per FR&SR then he can move to the CAT. Before doing so it is essential that sufficient time should have lapsed after his representation (giving full facts of the case ) to the competent authority
He can also submit an appear under Rule23(iv) of CCS(CC&A) Rules if he feels any rule is being distorted to his disadvantage. Such appeal will be addressed to the notified Appellate Authority or to the Authority higher than the Authority specified to Part V of Schedule to CCS(CC&A) Rules..