You need to colonize faster and build a stronger fleet. Instead of a Kol Battleship, build an Akkan Battlecruiser; it's Colonize ability is faster than Colony Frigates and boosts your economy considerably. Try to just aggressively conquer planets before your enemies can get to them.

The best way to defend a planet is with a fleet, and repair bays. They're both ultimately the cheaper and more effective way of defending a planet instead of turrets/hanger defenses.

Very small, that's what you should have within the first ten minutes of the game.

You can't count on defenses to hold back entire enemy fleets.  What that means is that you may need multiple fleets for different fronts.  Knowing where your enemies are who is threatening you is important.  Against the AI it's all about taking a little more ground every time you have some slack, then have a solid defense to prevent the AI from pushing you back.  You'll eventually wipe them out since the AI is a very poor attacker and needs you to slip-up in your fleet placement to win any real battles.

I actually don't know why beginners seem to all pick the Kol.  It's always been a rather mediocre capital ship, its only redeeming qualities being that it's hard to kill and it slaughters strike craft with flak burst.  If you want a tough damage-dealing monster, pick a Marza or Sova.

The gauss can put out some serious damage when level'd. Early game, if you can get two, you have serious fire power.

If possible, do your fighting on one front.  If not possible, you have to try to create chokepoints on your fronts and defend those chokes with well-placed static defenses and phase jump inhibitors backed by repair bays.  Yes, if you had entrenchment you could put starbases in the zone of your repair bays and defenses, which would certainly hold off 4 AIs provided you did it right.  Since you don't have entrenchment, you'll have to use what static defenses you have, and ships as well (always bugged me that ships had to sit at a planet using up fleet supply instead of fighting on my enemy's planets, thus entrenchment for me).

You might also try playing with vasari, and using their phase stabilizers.  Then all the jumping to and fro putting out fires in your empire is made relatively painless, as you can get from any planet to any other planet in one jump (provided of course you have installed phase stabilizers there).

If he's playing against AI and not human opponents he could build multiple Marzas, get them up to Level 6, and then just let the AI suicide itself on the missile barrages.

<Edit> It's even easier with Entrenchment because now you can upgrade the starbases and back them up with some hangers and repair bays and just let the AI suicide on the starbases.  (This works even better if you're Vasari and you build starbases with phase jump inhibitors.)