It could be medical or he could be maturing, I am not sure how old he is...
I am not sure if you do this but if either one of my dogs come to me for love, I ask them to sit or down before I pet them. I pet them when I am ready, not when they want it
You also have two male dogs, which could very well be frustrating to Zeus as Mr. Grey matures he is more of a threat to him...
Make sure you know if Zeus growled at you or not something else...
I thought one day, my CO growled at me but it turned out through observation it was 100% Zero that she growled at...because he got too close to her breakfast...wh ich i took care of that issue fast and they are fine eating next to each other in their crates
If it is not medical related, then it is something that you should spend a lot of time observing and paying attentions to both dogs body language...you may not see it but grey maybe challenging him
Good Luck
remember, dont be nervous because they (dogs) will sense that...calm and confident is how you need to be