I think that needle dropping when he’s going down hill is a clue: the thermal sensor is along the top of the block and that sensor does not behave when it isn’t fully emerged in coolant, as it would be with an air bubble in the system. When you roll down hill, that bubble shifts and the sensor is fully emerged, causing the gauge to go down. IOW, that sensor needs to be submerged in coolant all the time.
Like I said: the first thing to try is burping the system. On some cars, when you do refill the coolant, it is best to take the top radiator hose off, stick a funnel into it and fill the block first. Then, when it is near the top of the hose, reconnect that hose to the radiator quickly and then fill the rest of the system by means of the radiator.
Usually that prevents air bubbles.
Oh, and Hondas do not take just any coolant. You do have to be careful with the type you pick
Since our OP clearly isn’t very familiar with coolant but may be willing to try burping the system himself, I suggested he go to the dealer to get it, to make sure he gets the right stuff.