Alec wasn't an expressive person by nature. Reid was aware of that fact so Alec knew he didn't have to worry about his questions and his lack of huge grins and clapping weren't being misconstrued. Alec was happy, ecstatic for the both of them but he tended to show it in a different way.

"I -- " He was nearly speechless and he had to take a moment, swallow, work his jaw a bit to get the words out. "Yeah, I'd be honored to do whatever you need me to do including be your best man. I've -- "

He trailed off, laughed a bit ruefully. "I've never been to a wedding, Shadowhunter or otherwise. I might need to do some research to see what a best man even does."