I'd like to give you some suggestions as to how to resolve this. We all know the rule is illogical and poorly thought out, but here's my idea on what you could do.
1) Remind the DM that Cohorts (mention that) aren't considered as part of the party when calculating xp even though they get xp. Refer him to page 104 of the DMG where this rule is described in full. Therefore, its illogical that a summoned monster should take xp from the party when a Cohort, who is better, learns, levels and is more powerful, doesn't.
2) Suggest to the DM that since your summoning spells are effectively impaired below the intent of the game designers and game balance, suggest that you be allowed to keep track of the xp your summoned creatures get. Since they will be lower level than you, remind the DM that they will go up in levels faster than you. Set aside a write-up sheet complete with skills and feats, etc. Then ask him, "Are you SURE you want to give them xp?"
3) If the DM isn't keen on any of this, suggest to him that a better solution might not be to "NERF" your party (this is a PARTY PENALTY...not a CHARACTER PENALTY). Recommend instead that it would be more effective to better prepare his encounters.
RANT INCOMING: I am a FIRM believer in that there very few cases of an unbalanced power, item or ability. Just because a +1 Ring of Protection is more powerful than a ring made of bundled straw doesn't mean it's unbalanced. If a level 2 fighter takes a core feat that gives him 50 attacks in a single round...fine. It's only UNBALANCED if he's the ONLY ONE in the game that has that available. So when the displacer beast attackes every member of the 6 man party 8 times, no one is surprised and the game is effectively balanced. If everyone in your game has a +5 sword...it's okay...the monsters do to, negating any imbalance. I know I'm exaggerating quite a bit, but...suggest to your DM that a better solution would be for him to utilize summoning with some of his monsters, thereby negating any perceived imbalance. It will make the fights harder but...if the fights are being made easier by your summoning...shouldn't the encounters get harder? Thus...BALANCE is restored. The DM's job is to scale the monsters and conflicts so that they are challenging to the party. Some DM's, quite frankly, aren't very good at this job. It is SO not hard. I'm guessing it's never occured to your DM to just toss in another ogre into the monster group....
4) If your DM won't budge...don't worry about it. Just adapt and when you level, ask him if he'll let you "rebuild" your character (PH2, pg 192) since the recent change has heavily impaired your party under the pretext that you would have NEVER have built your character in that way if you knew how harmful to the party it would be (use the words "impaired" "penalty" "burden to the party" "negative" "harmful," etc). Heck, ask him if you can do it NOW. If he won't do that...ask to change characters so as not to be a PENALTY to your party simply by being in it. If he won't do that...find another DM.