I think you are overthinking this. A slight stammer is very common in young children, even monolingual ones, when they want to express more than their brain can keep up with; it doesn't mean they are going to end up with a permanent speech impediment.
Just be relaxed around his language, don't be horrified if he uses the occasional German word or even lapses into German for a specific situation; it doesn't mean he will never speak English again. Let it be something he feels good about, not bad.
Mine have always tended to go into majority language (English in our case) when recounting incidents that happened at school. They will still happily revert to Swedish for other purposes. And since they are 12 and 16, any language loss should probably have manifested itself by now. It hasn't.
I have posted this on the "Living in Germany" thread in Living Overseas, but it is a very bilingualism specific problem so I wanted to re-post here for a wider response, if that is OK.
We live in Germany and speak English as a family language, but pretty much 100% of the children's lives (the extended family they see regularly, Kindergarten, school, friends, sports, hobbies) happen in German. DH is German but his English is genuinely native speaker standard, aside from the very, very rare subtlety, but most English speakers think he is from an English speaking country.
My kids are 7.5 (DD), 5.5 (DS1) and almost 2 (DS2). We moved here shortly before the 5 year old (DS1) was born, and he is the one I am worried about.
We speak English at home almost exclusively - if the DC have friends here they speak German to them, but I still speak English to my DC and expect them to reply in English - actually I insist they do. I speak German to their friends, and DH speaks German to the kids when they have friends here, but 95% of the time English otherwise, even if I am not in the room. They speak German all the rest of the time though, like a lot of, or maybe all, the kids over about 3 on this thread pretty much everyone they see day to day is/ speaks German.
The language switching has never been a problem at all for DD, but I can see that it is becoming one for DS1. His German is definitely becoming stronger than his English, where DD's English has so far kept pace (she loves words and asking about meanings of any and every word she doesn't understand when I read to her at night etc. but DS1 either just lets the words of a bedtime story wash over him without bothering about understanding as long as he gets the gist, or reverts back to choosing picture books)
Anyway, the thing is, DS1 has started to stammer Only in English. He is easily able to identify that he does it when he can't think of the English word - it happens mainly when he is re-telling an anecdote that "happened in German" - at dinner he was telling a story about wanting to close the window at Kindergarten, and he started to stammer because he had forgotten the English word "window" shock It doesn't seem to worry him atm but DD started to tease him and I nearly bit her head off
WWYD? I cannot stand the idea of my kids losing their English and only understanding it not speaking it, and ending up as German speakers who speak good English - I know its what happens a lot in our type of situation but I just can't accept it, hate the idea, makes me want to pack them up and move back to the UK... It's irrational but even thinking about it makes me want to scream and ties my stomach in knots. But I also can't give my 5 year old some horrible speech impediment because I insist on making him speak English when, I am loathe to see it but have to make myself see, he probably thinks in German