RomeGirl will most likely be able to answer this question better than I can but I'll give it my best effort. An Equerry is a seasoned senior military officer. To reach that level of secondment, one must have served in good standing for many years. As well as serving in the Royal Household, the Equerry position is designed to deepen the knowledge and skills of an officer in preparation for further advancement in his branch of the military. The gentleman in the picture, as RomeGirl mentioned, appears to be very young. He has not been in the military long enough to have earned Jubilee medals. He does not have a "Long Service" medal. He is not wearing battle medals like Jonny's so he has not seen deployment action in a theater of war such as Iran or Afghanistan. To my untrained eye, it appears that he is too young in age and lacks the depth of military experience to have been selected by the King as THE Equerry. Since he is from the Coldstream Guards he is, speculating, the new temporary Equerry who will be with the King when Jonny, THE Equerry, is not. While the picture of the new Equerry is not in focus, it appears that his shoulder pips are all the same size and standing at the same elevation above his shoulder epaulettes as opposed to Jonny's shoulder decorations which differ in size and height when looking at them in profile which leads me to believe that the new person is a Captain.
On another note, the next time you see Jonny wearing his battle bars rather than his medals, notice the ribbon bar on the left of the second row. It bares a rosette which is an honour for serving with distinction and valor! As the French would say, "Chapeau" (hats off) to Jonny for his military abilities and actions!