Just spoken to my daughter and asked her for any tips for you. When they went to Corfu, they actually bought Sam a seat on the plane so that he would have his own luggage allowance - and even then they were overweight!
Some of the things she said she could have done without were nappies - very easy to get over there - and a bottle heater - she used a pan to heat his bottle. She also took lots of clothes for him to wear in the evenings and then found that she was putting him in his PJs before they went out at night, so that they wouldn't need to wake him up to change his clothes when they got home, so he only wore a few t-shirts during the day.
Another non-essential heavyweight item was a fortnight's supply of ready-mixed formula! She was unsure whether she would be able to get the brand that he was used to and thought that taking ready-mix would save a lot of hassle, but she now thinks that carrying the extra weight was probably not worth the small inconvenience of having to mix it fresh each time from powder.
One thing that they didn't take with them, but bought as soon as they arrived and which they found absolutely invaluable, was a small blow-up child's dinghy. As well as its intended use, it also acted as a bath, a paddling pool and a play pen for him.
Other useful things were mozzie nets for the buggy and cot and one of those net bags which are meant for slinging over the back of car seats, but which they used on the plane for keeping all his bits and pieces together that they needed for the flight.
She's promised to have a think about it and, if she comes up with any more ideas, I'll let you know.