A retired member or former member of a uniformed service, or a member of the Fleet Reserve or Fleet Marine Corps Reserve, who is serving on active duty is entitled to the pay and allowances to which he is entitled, under this title, for the grade, rank, or rating in which he is serving. In addition, while on active duty, he is entitled to the pay and allowances, while on leave of absence or while sick, of a member of a uniformed service of similar grade, rank, or rating who is entitled to basic pay.