Prepaid or other for teen traveling to US

My grandson, now 14, lives with his mom in Europe. He visits the US periodically and will, in the next few years, be graduating from flying as an accompanied minor with the airlines to traveling independently. He has a cellphone with a European plan that is expensive to use in the US.

We’d like to provide him with a prepaid phone and a US plan to use while here and while traveling in case he gets hung up and needs to contact us urgently. I’d like it to be a smartphone so that we can communicate via a variety of methods. He won’t need it in Europe and we wouldn’t want it to be racking up charges there in any case.

Any suggestions on phone, carrier, plan? Thanks.