Congratulations on your travel dates.
I remember this time last year waiting for our travel dates and while it was tough not to be able to travel before Christmas it was such a relief just to have a plan. It is definitely worth the few extra weeks wait
No specific tips for travel in the north as we were in the south, but we were there in February. Make sure to bring snow and rain gear, as we encountered lots of both. If you know your son's height and weight, I would consider bringing boots and a coat from here as they were much more expensive in Poland. We did bring coats and snow pants, but didn't have boots or shoes.