"Protestantism" is rather broad, but generally "his commandments" would be all of God's commandments for Man that weren't given as part of a specific covenent.
For instance, prohibitions against drinking wine or cutting hair were binding only on Nazarites.
Similarly, circumcision was commanded of the Israelites as part of the Old Covenant, as a physical symbol of the special relationship between Israel and God.
That God had extended his relationship to all of mankind under the New Covenant, and that Gentiles were not expected to receive circumcision was a concept that took the new Christians a while to understand. Acts 15 describes some of this process.
On the other hand, many of God's commandments apply to all mankind, and for all time. They are often presented as being related to God in some manner (e.g. sacred to God, or and abomination to God), and not a specific directive to a specific group of people. E.g. the Sabbath was declared a holy day during the first week of the Creation.
Deciding which commandments are for everyone, and which were for specific times and people, is one of the reasons there are so many denominations. The Adventist Churches say that Christians much obey the seventh day sabbath and clean meat laws, while on the other extreme some denominations say that Christians are no longer bound by the Ten Commandments.
As I said, "Protestantism" is rather broad.
As for God's commandments not being "heavy", John is pointing out that for a Christian, obeying God should be a natural thing, something that gives one pleasure, not something that could be considered a burden.
I once wrote something describing how the Ten Commandments could be seen in a totally different light, viewing them instead as Ten Freedoms.
I think that is the message that John is trying to convey.