No personal experience with variflow sorry. I can see that it might be helpful, especially with the BF weaning.
Not all refluxers are sick of course, many have silent reflux, although in my experience you can still hear them gulping and swallowing if you know what to listen for. I just wondered as reflux is a well known cause of finding feeding difficult.
I guess the other thing to think about is if the bottles are OK

. Or whether the gulping and spilling is due to him taking in too much air whilst feeding? My DS does this sometimes, but again you can often hear him gulp the air and you know the fussing and spilling is about to come.
I think 4 days is enough time to have adapted to new flow really.