I'm currently playing my first LG character in a while, so I'm re-learning how to react in that manner
Asl yourself some questions. Read the alignment as given in the PH, then talk to the DM about how he sees it. I think it's interesting that 'Servant of Good' is considered a
flaw. So, I think that you'd need to discuss some points with him.
One of the things I do is consider actions of the character and those around him in terms of 'civilized' behavior. It's led to some interesting conflicts between him and the CG druid: the druid was perfectly OK with feeding some of the human bodies we found in one place to the local wolves to keep them off our backs. I refused, and demanded they be given at least some sort of burial. Civilized people respect the dead.
Some consideration for the established structure of law and order is needed. A 'conservative' approach, as it were. A Lawful character is ordered in his behavior, but not nessesarily rigid. Good will often take a precedence over Law; remaining true to the absolute letter of the law regardless of other considerations strikes me as more LN than anything else.
Just some random thoughts on LG:
Laws are good; they lend structure to society and provide a common set of rules by which to play the game of life. Those who fail to respect that often have chaotic disordered lives that more often than not result in more pain and mysery than they should.
Law combined with Good tends to promote the good of society over the good of the individual, but not to the point of suffering. A good person should always strive to reduce pain, suffering, inequality, and needless harm.
He will kill, but only when forced to, taking no pleasure in it, when other means are plainly not going to work. He probably won't start a fight, save where inaction will cause more pain and suffering than fighting.
When faced with a street tough with a knife, he'll try to simply brain him, tun him, and hand him over to the guards for whatever justice awaits him. LG is restrained.
Serving a Pelor-like deity, you need to find out the tenets of that deity, ASAP. What is expected of the clergy?
Alignment is also, as has been said before, not a straitjacket. LG, like any other alignment, has wiggle-room to it. A single act won't result in an alignment change; misjudgements occur. People screw up. A LG character will try their utmost to be better, though. They will actively work towards Good. They will promote the rational solution over the wild-assed plan. They will look more kindly on an organized and peaceful capital city than a lawless frontier town.
Take care not to make the common mistakes many others make. I've heard people say a LG person will not drink, will not swear, and other odd things. LG is not some hideous fun-house mirror of half-remembered Puritanical or Calvinistic doctrine.
On the question of the character practicing the Dark Arts. If he does such things in front of you, there's going to be conflict. There's just no way of getting around that. And it'll probably come down to you or him walking away from it.
Make sure it's you.
Find out if such acts are against the local law. More than likely they are. Nothing at all wrong with getting him stinking drunk until he passes out, then hauling him down to the local temple and tossing him in a very deep hole where his necromancer butt will stay until it rots.
If it's not against the law for some reason, well, it's still against any sort of moral law. Creating undead is an evil act and there's just no real way of getting around that, no matter how you rationalize things or dress them up. In fact, it says so right on the label.
Animate Dead -- Necromancy [Evil]. Again, though, check with your GM. There may be some compelling reason it's not, or perhaps his world just works differently for some reason. Get an explanation, though, and make sure it's consistantly applied.
If there is some reason you must work together, be ceaseless in pointing out that it's a wrong thing to do. Don't be snide and don't be blunt, but get the point across to the other characters as well that just associating with this person is not a good idea. You lay down with dogs, you get up with fleas.
I'm assuming there is some overall reason for having two such dissimilar people in the party, so I'd talk with the GM. All other factors being the same, having a Pelor-like cleric and a necromancer in the same group is going to lead to one of them not being around in short order.