ERJ-170 Captain reported the First Officer allowed the aircraft to get a bit slow and deviate below the glide slope momentarily on an approach to BOS.

Narrative

I was Pilot Monitoring; with newer First Officer (not low time but also not being used on reserve) was Pilot Flying. Multiple runway changes eventually leading up to 22L to hold short of 27. There was a lot going on with looking for traffic; briefing how BOS works; following a heavy 747; holding short and distance available. Everything was perfectly stable prior to 1;000 ft. and we were in VMC from the start of the approach. As we came over the approach end of the runway; we started sinking a little quicker than normal and I told the FO to level off. Auto Throttle went into RTRD and the FO noticed but did not push [the throttles] up quickly enough. We got one 'GLIDESLOPE' before the FO got the plane leveled.We landed normally and were able to stop prior to 27. The speed did not turn amber and the VSI was always within criteria. I could've flown this leg but we were headed back to EWR and I figured with all the slam-dunk approaches EWR had been doing lately; BOS was a better leg for the FO. The FO also said he'd been trying to pick up more trips to get more experience; but without any luck. I'm not entirely sure why we started sinking faster all of a sudden; but during debrief the FO said he may have accidentally just let go of back pressure momentarily as the Auto Throttle went into RTRD.