help for my golden retriever's eyelid

Hello,
My golden retriever will be 10 in november. He's staying a few weeks with
me in my apartment at school (usually lives with my parents). Yesterday
morning I noticed that his right eyelid (on the bottom) is kinda droopy.
It's not staying firm against his eye the way it's supposed to. I thought
maybe something had gotten into it (bc i try to keep my bills down by
keeping AC on low and using fans), so i turned the fans off and put the AC
on to cooler. A friend also suggested giving him something with
antihistamine in it to lower the inflammation. I ended up giving him a tiny
bit of medicine with antihistamine and lowering the temp. When i returned
home a few hours later his eye looked much better, and there was a little
wet trail on the side of his eye (this has happened before, sometimes his
eyes water). and it looked much better. This morning it looks the same as it
did before it got better. Any ideas on what this is? or how to more
permanently cure it? Thanks for your help.
Jillian & Duke (my golden)