I assume you do not have iPhones (which I believe have a messaging capability not available to thise of us with Android devices).
Traditional texting is linked to a telephone number, which most eSIMs do not include; some eSIMs include calling capability and would therefore allow texting, but they cost more.
Many folks use WhatsApp, which provides a messaging capability when one has access to Wi-Fi. The Wi-Fi-only limitation can lead to delays in receipt of messages.
For a trip covering Slovenia and Croatia, I think your husband will need either two separate, country-specific eSIMs or a regional eSIM that covers a bunch of European countries. The regional eSIM will cost a bit more, but we're not talking about a lot of money. I don't think I've ever used a regional eSIM.
I use eSIMs from Airalo. They're easy to buy in the Airalo app, and if your husband miscalculate his data needs, it's easy to top up the eSIM.
I've found the Airalo installation instructions reliable, but it is important to know that they can differ, depending on the country of coverage. If your husband ends up with one eSIM for Croatia and a second one for Slovenia, he should not assume the instructions for the Slovenian eSIM are the same ones he used for the Croatian eSIM. It's important to read through the instructions carefully; there will only be a few steps. I'm emphasizing this because on two occasions I failed at this hurdle myself.
If your husband runs out of data, buying a top-up package is very simple--he just picks the duration and number of gigabytes of data needed and enters his crefit card info. Therefore, it's not critical to be precise about data usage. You can take a look at the top-up packages available in the Airalo app and see how their prices compare to buying more data initially. The difference is likely to be only $5 or so.