I need DMing advice for a solo campaign

[GnomeGninjas]

What CR encounters should I use? How many monsters per encounter? Should I do anything about the random treasure table probaly not giving him treasure he can use?

[skycycle blues]

What level of optimization are you going to allow him and what sort of character will he be?

If he's low-op, then in any given fight against CR=his level, there's about a 50% chance of him dying, possibly more.

If he's pretty optimized, CR=his level would probably be a good benchmark for a decent challenge, possibly even too weak of a challenge, especially as he levels up.

If you want to throw multiple things at him, use an encounter calculator and try to make the encounter level be close to the CR that you're shooting for. Maybe a little bit less, because more enemies is usually more of a challenge than one, unless the more are useless in any numbers (like 100 goblins vs a level 20, with some raw 20s, they might win, but they really shouldn't.).

Also, if he's a low level caster on his own, anything that can reach him will kill him.

And you can always adjust random treasure to either be useful stuff instead or just ensure that he can sell it for at least most of it's worth.

[Gabe the Bard]

You could also give the player an NPC ally, like a pet, partner, or lackey, in addition to any animal companion or familiar they might already have as a class feature. If you're starting at a higher level, you could also let the player take the leadership feat and acquire a henchman.

[GnomeGninjas]

He is a level 6 archery ranger with natural bond and poor ability scores. He has access to all of the books he can get his hands on.He is Charisma 7 so leadership might not work.

[Gabe the Bard]

Well, at least he'll have his animal companion.

You can still let him hire some help, or even let a relationship develop so the NPC becomes a friend or ally over time.

[mootoall]

Remember that the usual idea of action economy working for him is now the opposite. Try not to pit him against too many mooks at once, since they'll whittle him down fairly quickly. Climactic, one on one battles now actually work, which is good.

[Roaan]

The biggest problem with being a solo player is having a single health pool. Say you throw your level 6 ranger against a lvl 3 orc wielding a greataxe. Your ranger has roughly 30hp (assuming he has one CON mod and rolls average) this level 3 orc charges in, and you accidently roll a natural 20. You cringe as you roll a nice confirm and start totaling up the damage. This orc has 18 STR (totally acceptable for an orc) which makes for a total of (1d12+6)x3. It doesn't take calculus to tell you that if you rolled above a 3, you just one shot your poor level 6 player with a level 3. (Don't even think about if he had power attack)

In short, try to think out encounters and do your best to make sure nothing has too much static damage or a very large crit multiplier. Maybe give your player a magical item that gives him 25% crit immunity. This wont break the game and it will help keep him alive in case you do accidently crit his squishy face off.

[deuxhero]

Give him a druid's AC. It should work like that anyway.

[ClothedInVelvet]

See what he does, that's going to determine much of what you do.

I tried to set up an encounter that would be wayyyy over my PC's level so that he would have to try another way. Instead he started looking for NPC allies, found a few who would help him out, and went to war.

PCs can do crazy things, especially when they don't have a party weighing them down.