At this point you might want to dig into the Nengo code for more reference. For example, here is how the Izhikevich model is implemented. Note the recovery parameter used to track additional state information between successive calls to step_math. In your case, such a parameter could be used to remember values such as ‘the time that each neuron last spiked’.
It may also help if you could give more detail about your model. Depending what information is needed by step_math, there could be several different ways of going about this (e.g., as a custom unsupervised learning rule, or by doing something similar to the below).