Swap out the product ID 6455 on line 3 of the code to match your free shipping product. Full Tutorial https://wpsites.net/free-tutorials/free-shipping-per-product-woocommerce/

free-shipping.php

add_filter( 'woocommerce_package_rates', 'wpsites_show_only_free_shipping_for_specific_product', 20, 2 );
function wpsites_show_only_free_shipping_for_specific_product( $rates, $package ) {
    $target_product_id = 6455; // 🔹 Replace with your product ID
    $product_in_cart   = false;

    // Check if the target product is in the cart
    foreach ( $package['contents'] as $item ) {
        if ( $item['product_id'] == $target_product_id ) {
            $product_in_cart = true;
            break;
        }
    }

    // If the product is in the cart → remove all shipping methods except free shipping
    if ( $product_in_cart ) {
        $free_shipping_available = false;

        // Keep only free shipping if it already exists
        foreach ( $rates as $rate_id => $rate ) {
            if ( 'free_shipping' !== $rate->method_id ) {
                unset( $rates[$rate_id] );
            } else {
                $free_shipping_available = true;
            }
        }

        // If no free shipping method exists in the zone, create one dynamically
        if ( ! $free_shipping_available ) {
            $rates = array();
            $rates['custom_free_shipping'] = new WC_Shipping_Rate(
                'custom_free_shipping',
                __( 'Free shipping', 'woocommerce' ),
                0,
                array(),
                'custom_free_shipping'
            );
        }
    }

    return $rates;
}