My cousin keeps glancing at me and then looking away when I catch him?

Last night all of our close family went out for a meal and my cousin kept staring at me with a blank face he kept looking away whenever I caught him. he did come over to where I was sat and he said opposite me and said your alright and I replied with yeah and then we were generally chatting and I seen his pupils got bigger? I know this is a weird question but he's never done this before? im 15 and he's 16.
My cousin keeps glancing at me and then looking away when I catch him?
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