I can't seem to be able to get hold of XX all of a sudden, and I'm quite concerned.
I wonder if everything is alright with him.
Could I please ask you to have XX contact me at his earliest convenience?
この場合、次のような言い方ができますよ。
ーI can't seem to be able to get hold of XX all of a sudden, and I'm quite concerned.
I wonder if everything is alright with him.
Could I please ask you to have XX contact me at his earliest convenience?
「急にXXさんと連絡が取れなくなってとても心配しています。彼は大丈夫でしょうか。XXに都合がつき次第連絡をくれるよう伝えていただけますか？」
to get hold of XX で「XXに連絡を取る」
at one's earliest convenience で「都合がつき次第」
ご参考まで！
Could you kindly ask him to contact me via phone or email?
こんにちは！
ご質問ありがとうございます。
ご質問について、いくつか言い方が考えられますが、例えば、
I haven't been able to reach him since a few days ago, and I'm deeply concerned. Would you happen to know if everything is alright with him? I apologize for the inconvenience, but if it's not too much trouble, could you kindly ask him to contact me via phone or email? Thank you.
『数日前から彼と連絡が取れなくてとても心配しています。何かご存じでしょうか。[お手数おかけして](https://eikaiwa.dmm.com/uknow/questions/5945/)申し訳ございませんが、電話かメールで連絡いただけるように彼に伝えていただけませんでしょうか。よろしくお願いいたします。』と聞いてみるのはいかがでしょうか。
参考になれば幸いです。
カナダ 
日本 
