The player hoofing the ball and the striker scoring these goals will learning nothing in doing this and his team mates will eventually get bored. Playing from the back and doing what you’re doing is the right approach and once off sides come into play your team will adapt quicker. I had that several times in the lower age group. Winning at all cost mentality isn’t the right aapproach, it really is short term gains for no real long term strategy. 

Remember a high line has less relevance when there is no offside however it does help you compact play and regain possession. Risk reward. 

I used to keep a defender back 1v1 situation as the defender will learn about tracking strikers etc but keep the others with a high line. I found my players now understand the essence of man marking a player/1v1 strength when playing in the higher age group.

At this age conceding goals shouldn't be something we are overly concerned with. As you note his approach will lose its value  as offsides comes in and as any physical differences even themselves out.

As a general suggestion (continue) to focus on 1v1s/2v2s and the idea that everyone has to help when the team is defending.

As specific point if you are asking your defenders to play high your goalkeeper should play higher as well and be encouraged to play as an outfield player when the opportunity presents. However this over egging the pudding at U9 and of course offside is only/real reason a high line can be an effective approach.