He's been playing with this toy since yesterday. I think he must really love it.
This is the only toy he's been playing with since last night. I think it's his new favourite toy.
この場合、次のように言えると思います。
ーHe's been playing with this toy since yesterday. I think he must really love it.
「息子は昨日からこのおもちゃでずっと遊んでいます。このおもちゃが大好きに違いない」
has been playing with ... ということでずっと遊んでいることが言えます。
ーThis is the only toy he's been playing with since last night. I think it's his new favourite toy.
「息子は昨夜からこのおもちゃでしか遊んでいません。彼の新しいお気に入りのおもちゃみたいです」
one's favourite toy で「お気に入りのおもちゃ」と言えます。
ご参考まで！
He loves this toy so much that he's been playing with it since yesterday.
ご質問ありがとうございます。
・「He loves this toy so much that he's been playing with it since yesterday. 」
＝昨日からこのおもちゃがお気に入りでそればかり触ってる。
（例文）Did he like the toy?// He loves this toy so much that he's been playing with it since yesterday.
（訳）おもちゃ気に入ってくれた？//昨日からこのおもちゃがお気に入りでそればかり触ってる。
お役に立てれば嬉しいです。
カナダ 
日本 
