Wa alaikum salaam wa rahmatulahi wa barakatuh
As a parent, you are rightfully concerned about your son’s behavior and wish to correct him in the best way according to Islam. There are a couple of things to thong about here.
To have fallen in love with a girl it might be assumed that he has been spending time with this girl already to have developed such strong feelings. At his age, with growing up and the influences of hormones this is easily done. However, we must make sure that he does not fall into the trap of shaytaan.
It is important to make sure that any contact he does have with this girl is done within the perimeter of what is permissible in Islam, therefore this time with her should not be alone. However, any time spent with this girl, even if there are others present could lead to strengthening these feelings.
There are 2 primary ways to treat this situation; either encouraging marriage to this girl to make any contact halal and allow him to satisfy his desires or to warn him about the dangers of what he is doing and advise him to stay away. This will most likely depend on his exact age. If he is an older teenage then marriage may well be the better solution, but if he’s younger, then maybe he is best waiting as he may just be feeling confused and controlled by his hormones.
Islamically, there are no restrictions on what age he could get married so either way it is permissible Islamically, however, many countries have certain laws regarding the age permitted to get married and therefore it may be more difficult. You also need to think about his level Of maturity. We know that in Islam marriage is a highly valued thing and provides many a protection, but if he is not yet mature enough to understand this then he will have a hard time fulfilling his obligations to a wife at this point.
If you feel that marriage is the best option for him, then the best first step you can make is to consult with her family about the matter. If however, you believe that marriage is not an option for your son right now, then you need to educate him about having relationships with girls.
So, he needs to know the reasons behind why it is not ok to be alone with other girls and why free mixing is not ok also as it can lead to the development of feeling like he has which can easily lead to haram behaviors. If this is something you can do yourself then do so, otherwise, anyone you feel he would be most receptive to, be it his dad or the local imam, should have this conversation with him.
The contents of discussing this with him can be strengthened by showing strong relations with Allah in the home and ensuring he is mixing with good people that will influence him positively. Crating a strong Allah fearing home will ensure that he does not behave I ways that he knows will be displeasing with Allah.
All of these things will be further strengthened if supported by other family members in the house too so that he feels more desire to join in and pay attnetu4to what has been said regarding the etiquette of being with girls.
May Allah guide your son on the straight path and grant him a righteous spouse when the time is right.