Whistle training

I have a 16 month old mix, we do two pips on the whistle for recall, it’s definitely not 100% I would say his verbal cue is better but he knows that it means to come back and does it 10 times out of 10 in the house. We’ve recently started trying to train a whistle stop, the method I’m trying is throwing a treat over his head and we’re at the point where he’s putting together that my hand in the air means I’m about to throw a treat so he is stopping. My issue is I’m struggling to incorporate the whistle as anytime he hears it he comes bolting back, I tried not rewarding it and instead going back to the drill but by the end he wasn’t coming back for even the two pips so I’m conscious of ruining whatever recall we currently have 🥲

I’m thinking I’ll just keep with the drills and wait a little longer until I know he really understands that the raised hand means stop and then incorporate the whistle but does anyone have any other advice?