Hi Guest you may have trained and trained your dog but unfortunately not enough.
If he is always put on a lead the minute you see another dog then the desire to go over and actually make an acquaintance will be even stronger because dog to dog socialisation is such a novelty and so the circle will continue until one day your dog could be injured by a dog that is somewhat less than thrilled by his approach or by the owner and unfortunately you could be liable to prosecution under the DDA.
If training is important you will MAKE the time to attend a class or arrange for 1:1 tuition.
It is vital, especially with such a large dog, that your dog does listen to you.
http://www.apdt.co.uk/list_trainers.asp This link will show you what trainers are in your area.
Perhaps in the meantime you could consider using a headcollar on your dog to enable you to have better control; there are many models on the market such as the dogmatic, halti, canny collar, pullstoppa, gentle leader, gentle controller and the dogalter.
Best of Luck