[jet]

Both Jay and Louise agreed with him there not to spend any money, they should have told him when they decided to spend their money to avoid him feeling guilty or embarresed when they appeared with expensive gifts and he had nothing.

You forget that Aaron said to them in the DR to go ahead and buy gifts if they wanted the glory, but that he was out. That was a very cynical thing to say and after that Jay and Lou had no obligation to tell him anything.

Further...Aaron saw Jay and Lou go into the DR and he knew full well they had decided to buy the gifts because when they came out he shook his head and said 'silly people' under his breath.
If he was concerned about being embarrassed or felt guilty he could easily have approached them and decided to contribute his share or bought a gift of his own. But he decided not to - instead he twisted things to his own advantage to get camera time, sulk, and play the poor hard done by victim (again!)