We find GoMo really good, its €12.99 a month and includes all data and messages etc.
Just make sure to install software so he cannot go onto over 18 sites or use your card to buy things online (my nephew spend €200 on amazon on his phone using my sisters credit card!)
We take our daughter’s phone off her at night so she is not on it too much – and we check her screen time weekly and if it gets too high, we make sure to take it off her more and get her out of the house and doing other things. They get addicted so easily, its really hard finding a healthy balance
Good luck with it!! Hope this helped a little