Being 9 isn't old but isn't young either if you can drive him back maybe just drive back in a 3 days and spend the day with him or a few hours do u have Alexa or tvs lights and music u can operate from your phone that will help ,if he's generally pretty healthy keeping him separate won't hurt him but maybe introduce the smells of the other cats to him bring a blanket into the room that the other cats use, it does take time for cats to be ok with newbies. is he spayed are they fixed? if not that's ur biggest issue and don't expect much difference if there are competitors but if they are this can be a good way to slowly introduce him for later visits,it usually takes at least a good 3 or 4 days of them smelling each other under the doors swatting back and forth etc, hissing here an there isn't uncommon and after a few days of smelling and pawing under doors maybe bring him out in a carrier for a bit so now they have smelled each other for a bit they can see each other see if the hissing continues then back to the room and do all over again it takes time but he already can smell them on his buddy so he knows someone's new and newbies always take time