Ah, a ness problem, glad to see the little guy is having fun, but knowledge is power, and the secret to ness is the 3 situations that newer players use.
Situation 1: ness is defensive on the ground and has range to his attacks.
EX: YO-YO on the up and down slam. Grabs, roll&dash and PK fire. they generally roll away from the opponent, constantly updating their spacing between the 2 players, spamming those attacks while you approach.
Situations 2: Ness is in the air, and does fast/power moves.
EX: D-air, U-air, forward Air..... anything ness does in the air.... if he's close to your body, and your not fox/pika/kirby.... its quick and dirty, normally followed with another.
Situation 3: Ness is agrissive on the ground
EX: Dash-A, Dash grab, tripple tap combo into grab. Probably picking up every item he can see.
Most good ness players break these cycles to their attacks/movements just by understanding the game more....... but if I'm guessing correctly, these 3 tactices are your problems with his use of ness. And here's a basic overview of the answer.
For situation 1: throw off his distancing. Try to strike shallow (like a couple ft away... not to far, but just a fake attack aimed at him) not intending to hit him, z-cancel it, then roll away. Do this 3 or 4 times, and don't allow him to hit you.... he will get frustrated and try to either change up his distancing, or he will start running at you ahead of time to compensate to your shallow movements. Then switch back, he will get confused, and you'll catch him off guard for a couple of lives. This is more of a mental game, but it shakes up players who camp, Ness is slow, and you can always run away from him. And if he's just rolling for movement, he is really slow.
Situation 2: He will be coming at you from weird angles, hitting you and winning most moments of conflict.
The problem with his air game though, is he has to touch your hit box for it to be good. So, don't stand in 1 place for too long. Keep retreating, and spam tap attacks (not slams) in the only direction he can come from. You'll be able to see his jumping paths pretty easy, and if you can't, just test out ness, and figure out the form to ness's jump....... it really is a weak point if you know how the movement path works.
Situation 3: He is highly aggressive on the ground ehhhh? Just jump around. He is slow, you really get to pick your battles with ness unless its a small stage like peach's castle...... but ness does better there anyways. Just keep jumping backwards until he stalls out and then just jump strait up and slam down on him.
Other then that, spam projectiles..... Ness's down-B is only worth it if its Samus's breakfrast, or a free heal, so just strick close enough to charge him for the meal, and he will have a hurt *** every time the check bounce.
I really love playing ness as a character, but his short-comings are notorious in his down fall. Find a way to trip him up, either mentally or tactically, and he will just be a little boy again.