When my son was 2 1/2 he sunk his teeth into a little girls head one day! He left a terrible, terrible mark. I was absolutely mortified. I put him in his room becuase I was so upset and then came outside and cried my eyes out! Later I realized he was finally getting his two year molars. That week was a terrible week.He was grumpy misrable and struck out at everyone. He was probalby feeling rotten.
If he is teething maybe you can make sure he has painkiller or a binky or whatever you fel comfortable with.
Personally I let my children have binkys untill their done teething to prevent this. The same son is getting his 6 year molars (late again at 8) and he told me, "Mom I feel like I want to just bite on something!". Luckily hes old enough to control himself.
Also, check into his nap routine. Children this age still need between 12-14 hours of sleep. Is he getting this? Does his nap time need changed to a different time? Maybe he is getting tired when you go the park but becuase of his energetic nature your missing it. Read "Sleepless in America" by Mary Sheedy Kurckinca for tips on how to tell hidden signs of tiredness that we miss as moms and ways to actually get them to nap and sleep for the time their supposed to.
My feeling is you still have to let them know its wrong not matter what the reason is but as parents we need to be vigilant and find ways to prevent the behaviour.
Knowing that he has a hitting problem you have to follow him around the playground. You have to be right there. You cant leave him unsupervised for a second. This is a pain for you but it will make sure the other kids are protected.
Kids this age need constant repeitition. They need to know that it doesnt matter how many times they do the action they are going to get a consequence for it. You have to be consistent.
Let him know what you expect in his words before you go to the park. Talk to him about positive touch nad have him touch you or a teddy that way nad praise him. Tell him thats what we do with the other boys and girls. Have him practice at home on his brotehrs and praise praise praise!. Get your older boys involved by helping them set the example. Explain to htem what your trying to do and school them in how you want them to react if your 17 month old hits them. This may be agentle no no no, hands are for helping not hitting and immediately get your 17 month old to do something nice. He will quickly learn the joy of helping hands!
Also if you find your son raise his hand to hit antoher child when you are out I owuld suggest that that is when you should put him in time out. He cant even come close to hitting another child. If you have a stroller take him back to hte stroller, strap him in and give him his time out there. Just a few second or a minute so he doesnt forget why hes there. At the end remind him again to show you how to use his hands by doing something kind to you and then let him play again with you in tow. After his third warning/timeout tell him if he does it one more time we'll have to go home. This gives him a chance to do the right thing. Stick to your word. He will learn quiclky that mummy is going to be consistent aswell as learning the joy of beahving coreectly.
Sorry this is long but hope it helps