Building lean muscle is a long term goal mate, so just hang on in there.
As the upper half of your body gains in size it will effectively pull some of the skin and fat up with and tighten it up around your stomach area.
Training abs will only strengthen your core, it won't banish any fats or anything from it, so stick to your diet, keep getting yourself down the gym and one more suggestion...
You don't need 40 mins cardio, high intensity & short work outs will help you burn fat a lot quicker! Plus you have more time for the jacuzzi then!
Big congrats on how far you've come fella
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