Relentless lead biting and growling

Just had one of those walks, where you either come home crying or burst into tears as soon as you get in.

My 9 month old boxer teenager is relentless biting the lead on walks. He growls when he does it, but he isn’t trying to get at me (anymore) and I don’t think it’s aggression. I just feel like he’s so frustrated about something and I have no idea what.

It’s getting worse and more frequent – pretty much the entire walk he’s on and off lead biting or pulling like crazy.

I can’t figure it out and I’m at my wits end. Treats, his name, commands – nothing seems to snap him out of it other than standing on the lead for long enough that he gets bored and chooses to look at something else. Then as soon as we start walking again he’s pretty much back to lead biting. It’s embarrassing, sure, but I’m more so worried that I’m doing something horribly wrong to make him like this. I know he’s a high energy breed and he’s plenty of off lead exercise to run out and expend energy. But those walks around the block, or the walks to the field to let him off lead, are an absolute nightmare.

HELP

Edit: he gets plenty of rest, sleeps through the night in his crate no issues and naps for 2 hours or so at a time on and off throughout the day. We play tug in the house and the garden with rules (I.e. drop, wait, ‘ok’), we play a waited retrieve, we play ‘find it’ where he seeks out his kibble in the garden. He has licky mats, gets fed 3 x a day and recently upped his food to help him gain a little weight.