Receiving cut-off texts?

My boyfriend, who doesn’t want to replace his old POS cell phone, has only been receiving partial texts. No matter how short or long I make them, he only receives half of the text. Doesn’t always happen, but happens often enough that it irritates the shit out of me. It’s only been happening for the last month or so.

I text 10 other people during my day in the same way with no problem.

I have an iPhone 4S, he has an old samsung flip phone.

He claims it’s only my texts that do this. I live an hour away. He also receives texts from other iPhone users with seemingly no problems, but they’re in closer proximity.

Any clue what could be causing this?