I invited a Christian to Islam, and he started to pray and fast by way of experiment. Is there anything I should do, or should I let him try it out?.
There is nothing wrong with leaving him to try it out, whilst also reminding him of how easy this worship is and the great effect it will have on the soul, if he does it and is sincere towards Allaah, and that it is only prescribed for a great reason and not to cause difficulty for man. Here we see modern science speaking of some of the benefits of prayer.
He should also be reminded that it is a matter of individual taste and experience, rather it is a serious matter that has to do with attaining Paradise and being saved from Hell. And Allaah is the Source of strength.