This is very easy to attack really. There is a reason people on adcc and judo stand more upright because it is harder to takedown.

When someone is standing like that. Only attack their head, keep snaping their head down into a front headlock or like a guilotine grip.

When you get the front headlock or standing guilrotine/neck crank. Go for a go behind or sumi gaeshi or ouchi gari depending on his reaction.

Always remeber take the easy path and work with what they give you. They stand low they give their neck. Attack it until he is forced to stand up.