Ganun po talaga, magsi-sikreto yan kapag alam nyang hindi approve sa inyo. I don't have a teenager yet but I was young once. I remember nung sa first boyfriend ko, lagi ko sinasabi sa nanay kung nasaan ako, late na ko uuwi, etc. Nagagalit ang nanay, lalo na kapag pumupunta ako sa bahay ni bf. Once I told her "Ano gusto nyo? Magsinungaling ako sa inyo para hindi kayo nagagalit? Or sabihin ko pa rin sa inyo ang totoo kung ano ginagawa ko?" She admitted na prefer pa rin nyang nagsasabi ako ng totoo. I told her na magtiwala na lang sana sya sa akin at pinalaki nya ako nang tama.
Then I remember my classmate in college, nung nagka-bf sya, even against her parents' wishes, sobrang nakakatuwa. Doon ko narealize that having a relationship in school doesn't necessarily mean it would be a Distraction because in her case, it became an Inspiration ☺️ She graduated magna cum laude.
So ang gusto ko po sabihin, ay hindi talaga natin mako-control ang ating mga anak. Ang magagawa lang natin ay turuan at pangaralan sila. Like in your situation, I think it's better to address the fact na hindi tama that he's pursuing someone who's already taken. Rather than strictly forbidding him to have a gf, perhaps it's better to teach him what kind of girl he should be looking for. And reiterate the reason why ayaw mo pa sya magka-gf... As long as it doesn't affect his studies, and he doesn't get anyone pregnant, or cause whatever problems, there shouldn't be a problem right?
Anyways, I still don't know your whole situation and I could only imagine the hardships of being a single mom, but I hope kahit papaano I was able to give some ideas and insights that might work in your situation ☺️
Tere SC