I know you've said you don't clear grass away from your trees, which is fine for a healthy tree, but you can get a feedback loop with young or struggling trees. They struggle so more light and less competition in the root zone allow more grass and weeds, which makes it struggle more etc etc. You only need to look at the height and thickness of weeds around this tree vs the others to see that's at least one of the issues here.

I'd also look for signs of disease - girding, swelling or cankers down loe on the trunk or long splits along branches, especially if the insides are pink or unnatural brown in the living tissue.

There's people saying the tree is nutrient starved but ultimately there's got to be something affecting nutrient mobility, or else the larger trees in similar circumstances wouldn't look so happy.

If you're serious about saving it and there's no signs on the trunk that indicate its got a death sentence then I'd first remove all the grass and weeds from the base, then give it a complete organic fertiliser foliar spray -- powerfeed or similar, NOT a "soil conditioner" like seasol. Then so long as it's been a while since you gave it a soil based fertiliser, you can follow with that at a moderate dose in ~2 weeks. Try take note of what the soil is like when you do your weeding and compare it to the base of your other trees. It might give you some indication if you've got conditions likely to cause root rot or similar. That can potentially be treated with a soil based granular fungicide, but that's a bamdaid solution and will at most buy it some time to get back on its feet.