DD2 has an Islabike, and it is brilliant. With bikes, you get what you pay for and the fact that the second hand values are so good basically proves that these are decent bikes that hold their value.
We can't get DD2 off her bike, it is so light so she can properly enjoy going out on the bike and going for proper bike ride.
Yes, you can get bikes for under 100 quid but they are heavy and will last maybe a year or two before they are completely knackered. If you can afford that bit extra, go for it because your son will really benefit having a second hand islabike over a new thing from halfords or toys r us or tescos.
When our DD2 outgrows hers i would be surprised if we didn't get 50% of its value back anyway, so it will, in reality, only have cost slightly more than a toy bike anyway.
Worth every penny (can you tell I'm passionate about my bikes!!)