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With that plan, he could add weapon abilities to his bonded weapon at character level 13. Level 10 if he took the magical knack trait, level 12 with Prestigious Spellcaster, level 9 with both.

You only need the required caster level, not the wizard level.

Thanks, magical knack trait is taken. Good to know this only checks caster level.

Here is a tricky question which I could not find answer from rulebook or Google.

My team member has a lv6 Fighter and would about to take one level in Wizard at lv7 so he would be a lv6 Fighter/lv1 Wizard. And he plans to choose boned object feat which bonds his longbow as a lv1 Wizard the plan is to take next 10 levels (lv8 - lv17) as Arcane Archer, so his Wizard would always remain as level 1

Question is : could he enhance his bonded object: the longbow by adding expensive +4 or +5 equivalent special abilities when he is with high character level but with only as a lv1 Wizard? (according to rule: A wizard can add additional magic abilities to his bonded object as if he has the required Item Creation Feats )

Unchained or Regular? Unchained can't have Pounce at level 5. That aside, both a +2 Amulet and a +2 Belt are way over character wealth by level and not normal at all.

Regular.

Could use +1 Amulet instead so the wealth issue should be solved , new average damage becomes 71 which is still too much ( "average damage" here didn't consider the possible miss rate)

You say "average" damage, but +10 to hit at Level 5 is actually a little below average. The number of misses they'll have drops that DPR sharply. Also, yeah, both those magical items is fairly unusual for Level 5. Just the belt, maybe, which'd reduce damage and accuracy even further.

Summoner could take "Craft Wondrous Item" feat at lv3, and the cost to create "Amulet of Mighty Fists +2" is 8000gp , "Belt of Giant Strength +2" is 1000gp , 9000gp and 9 days to craft should be reasonable for a level 5 team.

To increase the chance to hit, could stop using power attack so the attack roll would be increased to 11 and the damage would be dropped to 63. There is also other way to increase the hi chance e.g. "bit of luck" from Cleric who get luck domain or "aid another" , both are cheap to get.

But the real problem is the huge advantage by comparing Eidolon and fighter in the same level , why a summoned monster could be even stronger than a "full-time employee".

Thinking of Eidolon of lv5 Summoner with below settings:

Items: Amulet of Mighty Fists +2, Belt of Giant Strength +2
Feats: Power Attack(-2, +4), Improved Natural Attack
Evolution pool: 12 points (Half-Elf Wild Caller), Extra Evolution * 2 (lv1, lv5)
Evolution (Quadruped): Limbs(Arm)*3, Slam*3, Pounce, Improved Damage(Slam)

So a battle would start with a charge and followed by a full-round attack:

Bite: +10, 1d6+10
Slam * 3: +10, 3d6+10

Average damage is 13.5 + 61.5 = 75

A quick look at monsters shows most monsters of cr5 would be one-turn killed.

Just thinking how much damage a fighter could produce each round in level 5? Mostly 20 or even less.

So is there any rule I am missing? or should add some house rule to limit the Eidolon with multiple slam attacks?

yes, it answer the question perfectly

Below is the description for "Aid Another" in http://paizo.com/pathfinderRPG/prd/coreRulebook/combat.html

Question here: is this action also treated as an "attack" because it said "roll an attack roll" or it only ends up with +2 bonus to AC or +2 bonus on next attack?

I guess the answer is the latter, because it said "distracting or interfering" with an opponent but not "attack". But I have to persuade GM who insist "aid another" is also an attack which would cause melee damage..

"Aid Another
In melee combat, you can help a friend attack or defend by distracting or interfering with an opponent. If you're in position to make a melee attack on an opponent that is engaging a friend in melee combat, you can attempt to aid your friend as a standard action. You make an attack roll against AC 10. If you succeed, your friend gains either a +2 bonus on his next attack roll against that opponent or a +2 bonus to AC against that opponent's next attack (your choice), as long as that attack comes before the beginning of your next turn. Multiple characters can aid the same friend, and similar bonuses stack."