Lots of good ideas and great posts here. Good for you for trying to fix the problem, too. I'm going to suggest a lot more exercise for your dog. Running/playing in the basement isn't really "exercise" for a dog...it's just playtime. Every dog needs a balance of playtime AND exercise, and lots of problems start when people confuse the two and/or neglect one. Even a 20 min walk once or twice a day would be a great start. Dogs NEED to walk and get out the yard or mental issues can start.
Even if you're trying other ideas already posted, try to commit yourself to at least one/two walks a day. Walk briskly and keep your dog at your side; no pulling and no stopping to sniff the flowers either. Walk like you're late for an appointment to keep your dog focused on you. If he's forging ahead, walk a square/box shape by turning left at 90 degree angles. Change pace - walk heel-to-toe slow, then back to your brisk pace. Do 180 degree turns whenever you think your dog is focused on something that's NOT you and don't wait for him to follow. Pivot on your right foot and immediately start walking away, no hesitation.
If you ride a bike or rollerblade - take him with you! Make him walk/trot/run next to you for 20 minutes and get him away from the property. The great thing about going that fast is that dogs usually can't focus on anything to start being aggressive about. If he sees a dog that he thinks he's going to attack - by the time he's thought about it, you're already past the other dog.
Just because he's a little dog doesn't mean he can't "work" like a big dog. My amazing dog walker walks 10 dogs, including a little chi for, more than hour on hilly terrain. That dog keeps up to the big dogs, no problem. Buy a little doggie backpack for your maltese and let him carry it on your walks then gradually add weight in the saddlebags. Start feeding him from an A-maze-ball/buster cube or put his kibble in a kong with some pb/mashed banana/water and freeze it for a few hours THEN give it to him for breakfast/dinner. That way he starts to use his mind and the more a dog uses his mind, the more tired they are. The more tired a dog is from physical AND mental pursuits - the calmer they are. Sounds like his mental/physical stimulation needs aren't being met. It won't "cure" the aggression but it will help big time.