Moving can be incredibly stressful to a pokémon, even if they’re intelligent enough to understand what you have to tell them. So the important thing to do first is to maintain some semblance of normalcy. Stick to your routine, even if you have a lot of packing to do. Walk Howlite at the same time every day, feed him on schedule, and so forth. This will help him cling to something stable, which in turn will keep him calm.
Second, believe it or not, you shouldn’t tell him you won’t leave him behind. Not in those words, anyway. Pokémon are intelligent, but they pick up on our tone a lot more readily than our words. If you say you won’t leave him behind, he’ll likely interpret this as you will leave him behind. Instead, say, “We’ll be moving together.” Talk about what you plan on doing in this new home. Describe his space in your new home. Perhaps even tell him a bit about the neighborhood and the things he can do in it if you happen to be moving a bit further out. Always maintain a positive attitude, and whenever talking about your new home, remember to include Howlite in the way you talk about it. Eventually, Howlite will understand that the new place will be his as well.
Finally, do everything you can otherwise to keep him calm. Walk him if he seems agitated by the activity. Play with him. Have a friend help you pack so you can devote some time to Howlite if you can. Then, on moving day, be sure to walk him first to help him release pent-up energy, then keep him with you at all times. Unpack his things as quickly as possible to give his space a touch of familiarity, and get back to your normal routine as soon as you can.
In short, be aware of Howlite’s stress levels and do everything you can to help him relax. Combine this with language that firmly establishes that the new place will be his (rather than that he shouldn’t worry about being left behind), and the move will go far smoother.
Best of luck, and congratulations on your new home!
The Pokédex According to Bill 