To do what?

I live in Pennsylvania and i was wondering if it would be legal for my husband and I

To do exactly what?

Until you give that info I can give an Unqualified... Yes... No... and Sometimes.

To give my son my husbands last name

To give my son my husbands last name

Yes, there are several way of doing this.

I assume your husband is not the boy's biological father?

We need more information to properly answer this question:

Who is the father?
How old is your son?
Who is shown as father on the birth certificate?

No my husband is not the bio dad

The best way to do it is for your husband to adopt. That normally requires consent by the "bio-dad".

In any case, the permission of the bio father will be needed
1. if it is merely a name change and not an adoption, permission of the bio father for the name change

2. adoption, permission for the adoption will be needed

But it is legal, but there is a very detailed legal method that needs to be done

... 2. adoption, permission for the adoption will be needed ...

Actually, Pennsylvania, as do other states, allows for involuntary termination of parentlal rights in adoptions (http://www.pacode.com/secure/data/231/chapter8015/chap8015toc.html). In such case, the father's permission would be dispensed with.