Most of the Med will still be nice in September - we always used to go then when dd was little. Italy, South of France, Spain etc. This place in the Languedoc is only £400 for a week in September, and you could get Ryanair flights to Bezier.
Went to Lanzarote with my DS last Feb - he was 18 months then. V family friendly, lots to see, a much nicer island than the other Canaries (in my opinion), should be sunny but not roasting hot in September. You can get great deals through Teletext holidays, tho since your DS is 2 you will have to pay for him now so the price you see online will prob go up when you phone to book it.
Also went to Santorini with DS the year before, also a great trip tho a lot of the old towns are not hugely buggy accessible. People v friendly tho, beaches really nice, food great and I think Easyjet flies direct from Gatwick.
We really liked Puerta Pollensa in Majorca when we went with our DS at the same age. The sea is calm and shallow with a little one.
We stayed at the Club del Sol aparthotel. The only downside is that it is slightly outside the main area so you need to get the bus or walk to get into town. I've added the Tripadvisor link below so you can check out the reviews. There may be better places closer to town though
Looking for a good value winter sun holiday but nowhere too hot (so not sure if canaries would be out for mid-late september) for me, DH and DS who will just be 2. Really flexible as where we can go so hard to know where to start looking and we want to go for a week-10days so shorthaul. We are based in Herts so can fly from Stansted & Luton and Heathrow/Gatwick if we have to.
Thinking of a apartment complex with a pool and maybe near a beach and within walking distance to restaurants etc. Also we may want to hire a car and see some sights. We will mainly be chilling out/swimming rather than wanting to sunbathe all day although probably not much chance of that now.
Any recommendations? Did start looking at the algarve and apartments and flights separately but flights seem pretty expensive coming back? Would consider anywhere if reasonable weather and nice location. Our budget for flights and accommodation would probably be a maximum of £1,500.