Coinbase, Kraken. Binance would probably not be suitable for you since it has a separate exchange for US citizens (Binance US) which afaik, has lower market volume compared to the global Binance one. I think an alternative that has copy trading would be Bybit Gemini. Turns out Bybit isn't allowed in the US as well.
Actually I think Binance is more suitable for him since I observe USA version of binance is gaining continuous reputation. Yes, I'm talking about the US version of it and they might have exclusive offers that are only available to the people residing on that country, which is great.
Coinbase and Kraken are big exchanges so maybe they do also have a version similar to Binance US? Hmm.. but too bad if Bybit doesn't allow US people because it might also be a good exchange as I'm seeing it across the forum lately. Gemini on the other hand must be a US-based exchange and I think they only allow people on the same country because I got a similar prompt last time upon trying to register on them.